I understand the concept behind _INCSEEK. Maybe my problem is that it is seeking 20,000 records or more. _INCSEEK = .50 didn't work for me. I don't know how fast you type or your users type, but typing 3 characters within .50 of a second is easy; not to mention that it really works out to be 1.5 seconds because you are typeing a couple of characters to perform the search.
I just think the users will find it painfully slow if they have to wait half a second between keystrokes.