Okay, I'm working on the 500 problem. I'll check back once I've got it fixed.
I'll take a look at FrontPage extensions and see if they work.
The server should be running much faster than before. Could you check Server Load under Server Status on the right panel in your cPanel and report what it is when the server seems to be running slow?
Here's an update:
The 500 problem seems to be caused by the server's use of ASP.NET 1.0 instead of 2.0. However, the Mono Project's directions for setting to ASP.NET 2.0 aren't working... instead, they seem to be disabling ASP.NET altogether. I'm contacting cPanel to see if they have any suggestions on this matter.
As for the FrontPage extensions, check if they work now.