I installed DirectX 9.0b(summer update) and everything seemd OK. But after that applications that depends on DirectX start to lag. Why is that, and is there any remedy? Should''nt DirectX be downward compatible?

Maybe you used the retail runtimes first and switched to debug when you installed the update?

More than likely, you selected the Debug Runtime when you installed the SDK (which is a good thing). The Debug Runtime will give you lots of very nice info about why DirectX calls fail.

But when you want to run an application at full speed and don''t need this debug information (like when you play a game) you can just go to Control Panel->DirectX->Direct3D and make sure that you select the Retail Runtime.

Using the Control Panel, you can easily switch between Debug and Retail Runtimes whenever you want. By the way, you should ALWAYS use the Debug Runtime when developing DirectX applications.

neneboricua