How can I fix the slow freezing text messaging app on my MyTouch 3G Slide?
Handcent SMS is the most oft-recommended SMS replacement app on this site, so it's probably good to try.  If you have no issues with it, then I would assume it's just a buggy SMS app that you have and not a deeper problem.

If you don't mind losing all your messages you could try going to Settings -&gt; Applications -&gt; Manage applications, finding the SMS app ("Messaging" or similar), and clearing the data for it. There may also be something like "Messaging Storage" that you need to clear.

I don't know of any app that will improve performance of the existing app, and replacing a messaging app can be tricky.  I'd recommend a custom ROM that has had some of these bugs worked out.  This XDA topic has a good overview of ROMs for your device.