I am running Exchange 2010 SP1 UR1 at server with 2GHz Quad-Core Processor and 5 GB of RAM and I have problems with slow loadtimes when I try to access to OWA. Especially when I go to OWA at the first time after server reboot. How can I improve OWA loadtimes? Can installing of Dynamic Content Filtering for IIS solve this problem?

Are you facing OWA slow issue after UR1. Did you test by disable IPV6 and then check. By seeing some article its looking that Exchange 2010 Beta versio was having OWA also issue and this is been resolved by SP1. Anil

The problem has been before installing SP1 already. There is manual how to improve OWA speed: http://justinho.com/post/Improving-Exchange-2010-OWA-Page-Loadtimes.aspx Can I do it?

I found that I have gzip compression enabled. Dynamic compression is also enabled.

Can you increase the RAM? You should have at least 8GB RAM for the CAS role. http://technet.microsoft.com/en-us/library/dd346700.aspx Understanding Memory Configurations and Exchange Performance