I find the performance of this emulator quite odd in many ways. It is definetely not a stable SDK. I ran it on fast 2GHz pcs and it worked very slow while on a 450MHz laptop it was just fine. Sometimes it does not even start at all.
I don't think its a matter of the machine youre using. This lack of performance is probably due to many bugs of the emulator itself.
After you use this emulator a few times in a row, check your task manager and check out if some "midp.exe" process is still running, since it often happens that the emulator process doesnt stop even after you you close it (especially if youre using the WTK - with JBuilder the process is almost always correctly stopped).