diff --git a/src/gl/renderer/gl_renderer.cpp b/src/gl/renderer/gl_renderer.cpp
index eab8fa0ff9..98aee05c08 100644
--- a/src/gl/renderer/gl_renderer.cpp
+++ b/src/gl/renderer/gl_renderer.cpp
@@ -558,7 +558,7 @@ void FGLRenderer::DrawBlend(sector_t * viewsector, bool FixedColormap, bool doco
 	{
 		if (!viewsector->e->XFloor.ffloors.Size())
 		{
-			if (viewsector->heightsec && !(viewsector->MoreFlags&SECF_IGNOREHEIGHTSEC))
+			if (viewsector->GetHeightSec())
 			{
 				auto s = viewsector->heightsec;
 				blendv = s->floorplane.PointOnSide(r_viewpoint.Pos) < 0? s->bottommap : s->ceilingplane.PointOnSide(r_viewpoint.Pos) < 0 ? s->topmap : s->midmap;
diff --git a/src/gl/scene/gl_bsp.cpp b/src/gl/scene/gl_bsp.cpp
index 83620cbb24..169b7513ca 100644
--- a/src/gl/scene/gl_bsp.cpp
+++ b/src/gl/scene/gl_bsp.cpp
@@ -497,7 +497,7 @@ void GLSceneDrawer::DoSubsector(subsector_t * sub)
 			// but undetermined heightsec state. This can only happen if the
 			// subsector is obstructed but not excluded due to a large bounding box.
 			// Due to the way a BSP works such a subsector can never be visible
-			if (!sector->heightsec || sector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC || in_area!=area_default)
+			if (!sector->GetHeightSec() || in_area!=area_default)
 			{
 				if (sector != sub->render_sector)
 				{
diff --git a/src/gl/scene/gl_scene.cpp b/src/gl/scene/gl_scene.cpp
index ef17b71282..99cb6e112c 100644
--- a/src/gl/scene/gl_scene.cpp
+++ b/src/gl/scene/gl_scene.cpp
@@ -103,7 +103,7 @@ void GLSceneDrawer::SetViewArea()
 	r_viewpoint.sector = R_PointInSubsector(r_viewpoint.Pos)->render_sector;
 
 	// Get the heightsec state from the render sector, not the current one!
-	if (r_viewpoint.sector->heightsec && !(r_viewpoint.sector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC))
+	if (r_viewpoint.sector->GetHeightSec())
 	{
 		in_area = r_viewpoint.Pos.Z <= r_viewpoint.sector->heightsec->floorplane.ZatPoint(r_viewpoint.Pos) ? area_below :
 				(r_viewpoint.Pos.Z > r_viewpoint.sector->heightsec->ceilingplane.ZatPoint(r_viewpoint.Pos) &&
diff --git a/src/hwrenderer/scene/hw_fakeflat.cpp b/src/hwrenderer/scene/hw_fakeflat.cpp
index 6b535bdad9..3b0c350429 100644
--- a/src/hwrenderer/scene/hw_fakeflat.cpp
+++ b/src/hwrenderer/scene/hw_fakeflat.cpp
@@ -160,9 +160,7 @@ bool hw_CheckClip(side_t * sidedef, sector_t * frontsector, sector_t * backsecto
 
 area_t hw_CheckViewArea(vertex_t *v1, vertex_t *v2, sector_t *frontsector, sector_t *backsector)
 {
-	if (
-		(backsector->heightsec && !(backsector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC)) &&
-		(!frontsector->heightsec || frontsector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC))
+	if (backsector->GetHeightSec() && !frontsector->GetHeightSec())
 	{
 		sector_t * s = backsector->heightsec;
 
@@ -189,7 +187,7 @@ area_t hw_CheckViewArea(vertex_t *v1, vertex_t *v2, sector_t *frontsector, secto
 //==========================================================================
 sector_t * hw_FakeFlat(sector_t * sec, sector_t * dest, area_t in_area, bool back)
 {
-	if (!sec->heightsec || sec->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC || sec->heightsec==sec) 
+	if (!sec->GetHeightSec() || sec->heightsec==sec) 
 	{
 		// check for backsectors with the ceiling lower than the floor. These will create
 		// visual glitches because upper amd lower textures overlap.
diff --git a/src/hwrenderer/scene/hw_sprites.cpp b/src/hwrenderer/scene/hw_sprites.cpp
index ad21180638..6e20960a1a 100644
--- a/src/hwrenderer/scene/hw_sprites.cpp
+++ b/src/hwrenderer/scene/hw_sprites.cpp
@@ -309,7 +309,7 @@ void GLSprite::PerformSpriteClipAdjustment(AActor *thing, const DVector2 &thingp
 				}
 			}
 		}
-		else if (thing->Sector->heightsec && !(thing->Sector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC))
+		else if (thing->Sector->GetHeightSec())
 		{
 			if (thing->flags2&MF2_ONMOBJ && thing->floorz ==
 				thing->Sector->heightsec->floorplane.ZatPoint(thingpos))
diff --git a/src/p_sectors.cpp b/src/p_sectors.cpp
index e0eca11dbb..b5f10b69ee 100644
--- a/src/p_sectors.cpp
+++ b/src/p_sectors.cpp
@@ -1068,30 +1068,6 @@ FSectorPortal *sector_t::ValidatePortal(int which)
 //
 //=====================================================================================
 
-sector_t *sector_t::GetHeightSec() const
-{
-	if (heightsec == NULL)
-	{
-		return NULL;
-	}
-	if (heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC)
-	{
-		return NULL;
-	}
-	if (e && e->XFloor.ffloors.Size())
-	{
-		// If any of these fake floors render their planes, ignore heightsec.
-		for (unsigned i = e->XFloor.ffloors.Size(); i-- > 0; )
-		{
-			if ((e->XFloor.ffloors[i]->flags & (FF_EXISTS | FF_RENDERPLANES)) == (FF_EXISTS | FF_RENDERPLANES))
-			{
-				return NULL;
-			}
-		}
-	}
-	return heightsec;
-}
-
 DEFINE_ACTION_FUNCTION(_Sector, GetHeightSec)
 {
 	PARAM_SELF_STRUCT_PROLOGUE(sector_t);
diff --git a/src/p_setup.cpp b/src/p_setup.cpp
index c718bdeaba..50733318ab 100644
--- a/src/p_setup.cpp
+++ b/src/p_setup.cpp
@@ -4221,6 +4221,23 @@ void P_SetupLevel (const char *lumpname, int position)
 		AnnounceGameStart ();
 	}
 
+	// This check was previously done at run time each time the heightsec was checked.
+	// However, since 3D floors are static data, we can easily precalculate this and store it in the sector's flags for quick access.
+	for (auto &s : level.sectors)
+	{
+		if (s.heightsec != nullptr)
+		{
+			// If any of these 3D floors render their planes, ignore heightsec.
+			for (auto &ff : s.e->XFloor.ffloors)
+			{
+				if ((ff->flags & (FF_EXISTS | FF_RENDERPLANES)) == (FF_EXISTS | FF_RENDERPLANES))
+				{
+					s.MoreFlags |= SECF_IGNOREHEIGHTSEC;	// mark the heightsec inactive.
+				}
+			}
+		}
+	}
+
 	P_ResetSightCounters (true);
 	//Printf ("free memory: 0x%x\n", Z_FreeMemory());
 
diff --git a/src/p_spec.cpp b/src/p_spec.cpp
index da20dec4af..3c140f5f78 100644
--- a/src/p_spec.cpp
+++ b/src/p_spec.cpp
@@ -1374,6 +1374,7 @@ void P_SpawnSpecials (void)
 				{
 					level.sectors[s].heightsec = sec;
 					sec->e->FakeFloor.Sectors.Push(&level.sectors[s]);
+					level.sectors[s].MoreFlags |= (sec->MoreFlags & SECF_IGNOREHEIGHTSEC);	// copy this to the destination sector for easier checking.
 					level.sectors[s].AdjustFloorClip();
 				}
 				break;
diff --git a/src/polyrenderer/scene/poly_sprite.cpp b/src/polyrenderer/scene/poly_sprite.cpp
index c5c487c9ac..12406b5c04 100644
--- a/src/polyrenderer/scene/poly_sprite.cpp
+++ b/src/polyrenderer/scene/poly_sprite.cpp
@@ -182,7 +182,7 @@ double RenderPolySprite::GetSpriteFloorZ(AActor *thing, const DVector2 &thingpos
 			return floorh;
 	}
 
-	if (thing->Sector->heightsec && !(thing->Sector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC))
+	if (thing->Sector->GetHeightSec())
 	{
 		if (thing->flags2&MF2_ONMOBJ && thing->floorz == thing->Sector->heightsec->floorplane.ZatPoint(thingpos))
 		{
@@ -204,7 +204,7 @@ double RenderPolySprite::GetSpriteCeilingZ(AActor *thing, const DVector2 &thingp
 			return ceilingh;
 	}
 
-	if (thing->Sector->heightsec && !(thing->Sector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC))
+	if (thing->Sector->GetHeightSec())
 	{
 		if (thing->flags2&MF2_ONMOBJ && thing->ceilingz == thing->Sector->heightsec->ceilingplane.ZatPoint(thingpos))
 		{
diff --git a/src/r_defs.h b/src/r_defs.h
index fd702c5964..2ab250d1d0 100644
--- a/src/r_defs.h
+++ b/src/r_defs.h
@@ -654,7 +654,12 @@ public:
 	void ClosestPoint(const DVector2 &pos, DVector2 &out) const;
 	int GetFloorLight () const;
 	int GetCeilingLight () const;
-	sector_t *GetHeightSec() const;
+
+	sector_t *GetHeightSec() const
+	{
+		return (MoreFlags & SECF_IGNOREHEIGHTSEC)? nullptr : heightsec;
+	}
+
 	double GetFriction(int plane = sector_t::floor, double *movefac = NULL) const;
 	bool TriggerSectorActions(AActor *thing, int activation);
 
diff --git a/src/swrenderer/scene/r_opaque_pass.cpp b/src/swrenderer/scene/r_opaque_pass.cpp
index a3e1b1bd97..a7ba00c0c1 100644
--- a/src/swrenderer/scene/r_opaque_pass.cpp
+++ b/src/swrenderer/scene/r_opaque_pass.cpp
@@ -554,8 +554,7 @@ namespace swrenderer
 		VisiblePlane *ceilingplane = frontsector->ceilingplane.PointOnSide(Thread->Viewport->viewpoint.Pos) > 0 ||
 			frontsector->GetTexture(sector_t::ceiling) == skyflatnum ||
 			portal != nullptr ||
-			(frontsector->heightsec &&
-				!(frontsector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC) &&
+			(frontsector->GetHeightSec() &&
 				frontsector->heightsec->GetTexture(sector_t::floor) == skyflatnum) ?
 			Thread->PlaneList->FindPlane(frontsector->ceilingplane,		// killough 3/8/98
 				frontsector->GetTexture(sector_t::ceiling),
@@ -597,8 +596,7 @@ namespace swrenderer
 		VisiblePlane *floorplane = frontsector->floorplane.PointOnSide(Thread->Viewport->viewpoint.Pos) > 0 || // killough 3/7/98
 			frontsector->GetTexture(sector_t::floor) == skyflatnum ||
 			portal != nullptr ||
-			(frontsector->heightsec &&
-				!(frontsector->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC) &&
+			(frontsector->GetHeightSec() &&
 				frontsector->heightsec->GetTexture(sector_t::ceiling) == skyflatnum) ?
 			Thread->PlaneList->FindPlane(frontsector->floorplane,
 				frontsector->GetTexture(sector_t::floor),
diff --git a/src/swrenderer/things/r_visiblesprite.cpp b/src/swrenderer/things/r_visiblesprite.cpp
index 458e83dbb0..cc9ce8825d 100644
--- a/src/swrenderer/things/r_visiblesprite.cpp
+++ b/src/swrenderer/things/r_visiblesprite.cpp
@@ -173,7 +173,7 @@ namespace swrenderer
 			hzb = spr->gzb;
 		}
 
-		if (spr->heightsec && !(spr->heightsec->MoreFlags & SECF_IGNOREHEIGHTSEC))
+		if (spr->heightsec)
 		{ // only things in specially marked sectors
 			if (spr->FakeFlatStat != WaterFakeSide::AboveCeiling)
 			{