Like def said, your execution speed through IB will be determined by your connection speed to the internet. TMBR and ISLD are definitely under half a second, from the time you hit the key to the time you see the shares in your account, probably even faster, but I can't really look that fast. My best guess would be an eighth of a second. Instinet is around one second, NYSE anywhere from a second to a couple of minutes. Your typical liquid stock like GE, GM, DD, EK will most likely take 5 to 10 seconds for a marketable 1000 share order under average market conditions.
In very heavy volume, IB's quotes sometimes lag up to a few seconds, but executions are still quite fast (about one to two seconds for ISLD, 10 seconds for other ECNs) Most of that delay is probably caused at the exchange itself.