kconfig.debug: mention that DEBUG_SLAB can slow down machine quite a bit

I experienced x3 slowdown due to this option being set.

Please add this small warning to DEBUG_SLAB help text.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

lib/Kconfig.debug

@@ -34,7 +34,7 @@ config DEBUG_SLAB
 	help
 	  Say Y here to have the kernel do limited verification on memory
 	  allocation as well as poisoning memory on free to catch use of freed
-	  memory.
+	  memory. This can make kmalloc/kfree-intensive workloads much slower.
 
 config DEBUG_SPINLOCK
 	bool "Spinlock debugging"