perf(mallocfree): only poison freed V blocks at --sanity-level=3

coregrind/m_mallocfree.c

@@ -2138,8 +2138,18 @@ void VG_(arena_free) ( ArenaId aid, void* ptr )
       carefully chosen junk :-), in that: (1) 0xDDDDDDDD is an invalid
       and non-word-aligned address on most systems, and (2) 0xDD is a
       value which is unlikely to be generated by the new compressed
-      Vbits representation for memcheck. */
-   if (aid != VG_AR_CLIENT)
+      Vbits representation for memcheck.
+
+      CodSpeed: this is a pure debugging aid (it only makes use-after-free
+      bugs *in V itself* more likely to be noticed), but it is expensive:
+      it writes every byte of every block V frees, which on a debuginfo
+      heavy run amounts to hundreds of MB of stores plus the page faults
+      needed to first-touch the parts of a block that were never used.
+      It is therefore only done at the sanity levels reserved for
+      expensive self-checks (--sanity-level=3 and above); the matching
+      fill on the malloc side is disabled by default for the same
+      reason (see VG_(arena_malloc)). */
+   if (aid != VG_AR_CLIENT && UNLIKELY(VG_(clo_sanity_level) >= 3))
       VG_(memset)(ptr, 0xDD, (SizeT)b_pszB);
 
    if (! sb->unsplittable) {