A couple of trivial perf improvements:

-BMEMSET macro should cache the size of the memory block instead of reading each iteration
-Avoid tesselating the backup copy of the mesh used to restore after an error (unless there is actually an error)

source/blender/blenkernel/intern/editderivedbmesh.c

@@ -109,10 +109,17 @@ BMEditMesh *BMEdit_Copy(BMEditMesh *tm)
 	*tm2 = *tm;
 	
 	tm2->derivedCage = tm2->derivedFinal = NULL;
-	
-	tm2->looptris = NULL;
+
 	tm2->bm = BM_Copy_Mesh(tm->bm);
-	BMEdit_RecalcTesselation(tm2);
+
+	/*The tesselation is NOT calculated on the copy here,
+	  because currently all the callers of this function use
+	  it to make a backup copy of the BMEditMesh to restore
+	  it in the case of errors in an operation. For perf
+	  reasons, in that case it makes more sense to do the
+	  tesselation only when/if that copy ends up getting
+	  used.*/
+	tm2->looptris = NULL;
 
 	tm2->vert_index = NULL;
 	tm2->edge_index = NULL;

source/blender/blenlib/BLI_utildefines.h

@@ -247,6 +247,6 @@ do { \
 #  define BLI_assert(a) (void)0
 #endif
 
-#define BMEMSET(mem, val, size) {unsigned int _i; char *_c = (char*) mem; for (_i=0; _i<size; _i++) *_c++ = val;}
+#define BMEMSET(mem, val, size) {unsigned int _i, _size = (size); char *_c = (char*) mem; for (_i=0; _i<_size; _i++) *_c++ = val;}
 
 #endif // BLI_UTILDEFINES_H

source/blender/editors/mesh/bmeshutils.c

@@ -156,6 +156,7 @@ int EDBM_FinishOp(BMEditMesh *em, BMOperator *bmop, wmOperator *op, int report)
 
 		BMEdit_Free(em);
 		*em = *emcopy;
+		BMEdit_RecalcTesselation(em);
 
 		MEM_freeN(emcopy);
 		em->emcopyusers = 0;