What does your code look like in TestComplete for interacting with your grid. There are good ways and bad ways of doing things when it comes to performance.
Someone else will have to come along and comment on whether or not TC has problems with performance with such controls. I do know that, in the past when I worked with a Delphi application where I needed to use debug info, the first time in a test run it needed to access the debug info it took a hit in timing... but, within the test run after that first point, everything else worked just fine.