Neither Find/Replace nor RegexPal search the full history of a project, so I can't imagine having a long history would slow things down.
The general answer is that you can't edit or trim the history in a project. The more technically accurate answer is that you can't do it with your current project, but you can create a new project which has things trimmed down. You have to keep in mind that that requires the entire team to delete the old one and re-download a new one, and that you'll have to register the new on on the PT Registry. It's a question of whether it's worth the time and trouble.
The actual process of trimming history is also complex, involving command line tools and the actual versioning program PT uses (Mercurial). If all that sounds like a foreign language, you'd need technical help if you wanted to do this. If that sounds possible, I can search my notes for the actual commands I used a while back to delete history older than a certain point (I did this when we were moving from PT 7 to 8, and thus re-registering the project anyway).