I hear ya. Sorry, but I myself have found no way to improve this. It is embarrassing training new users, and waiting an eternity for the dialog to open. I noticed it is much better in 12 though (50% better I'd say). Still aggravating.
Was it at one time fast on the same computer you are on, and then got more sluggish? I have noticed general consistency on a given computer, that over time gets slow. I then I delete the .mtx file as you did and it gets a little better.
I remember filing IRs about it years back, but you can only scream into the void so much before you become silent.
@Kal Yea, but I try not to sweat the small stuff. I had to use Mastercam at one job, and NX is still far superior. The small complaints I have with NX are nothing compared to the nightmare using Mastercam was. It had nothing to with lack of experience either, it was literally 20 clicks to do simple tasks. Mastercam's poor graphics, glitchy interface, and inability to handle large files drove me crazy.
So now I multitask, and sip my coffee during the extra second the dialog takes to open