Task Managers.
They're a pain. The creators of Windows Mobile (Microsoft) decided that we didn't actually want to shut our programs down after use - we wanted to minimise them.
Hence after 30 mins of usage your phone is slowed to a standstill.
A lot of people have created programs to remedy this, I used to use Tman for example, but HTC have created a really, really solid one.
And here it is - the new version of HTC Task manager
Task Manager Help
In the options you can set programs to shut with a click - it works great.
Enjoy and let me know how you get on with it!
Here's a how to -