We already investigated this problem as eltonik's company purchased a Premium Support pack.
We suspected a low bandwidth problem or high processor or memory usage caused by some running processes. The problem was actually incorrect DNS configuration. EPESI was working fast, only when opening Roundcube mail client and fetching messages from home.pl server it became slow.
In this particular case the problem was solved by the user not by correcting DNS configuration but by creating a manual entry in hosts file pointing to home.pl server (not the best solution but it works).
For future reference - you can figure out the best DNS server for your network (if you do not maintain your own) by using this excellent utility: http://code.google.com/p/namebench/
If you don't want to spend much time looking for the best DNS server you can safely use Google's public DNS servers:
8.8.8.8 and 8.8.4.4
They are fast, public and easy to remember 🙂
https://developers.google.com/speed/public-dns/