There are various different speeds of coin merged-mined alongside bitcoin, including one (GeistGeld) that is so crazy fast that some pools think it is too fast to be effectively merged mined, thus too darn fast to be secured.
DOGE recently showed us that almost all the GPU-mined coins could be PWNed overnight just by some stupid internet meme. DOGE itself only has as much hashing power as a stupid internet meme can, almost overnight, conjure up. So it and all those that have less GPUs dedicated to securing them are all just garbage, toys, sitting there waiting for anon or some internet meme or who-ever or whatever to PWN the lot of them.
So if you want really crazy fast, help determine whether GeistGeld is too fast to work, and if so maybe push to have it slowed down to whatever is the fastest that can actually practically be secured. Meanwhile check out the other merged coins, some more of them are much faster than Bitcoin but not so stupidly fast as to be impractical to secure...
-MarkM-
From when on does the security becomes impractical?