As far as I know, there is no way to change the form search functionality. We have the same problem you do. One suggestion I will make is to take advantage of the relatively new "smart folders" feature as much as possible. (You will need to have Machform 13 or higher to have this feature.) It is much faster than "interactive searching" and if you use a "smart folder" to limit the number of forms displayed first, then "interactive searching" can become much faster.
I am not saying this is a replacement for usable search functionality. It isn't. Appnitro really needs to either make the "interactive search" in the "Form Manager" work at a reasonable speed, even with a large number of forms, or change it to wait until you have typed the full search string and pressed return or clicked a button to perform the search, instead of doing it after each key press; just as you stated above.