I play on the same computer that hosts the dedicated server, and I was having lag issues, but I found a way to fix it (most of it that is)
1.while both the dedicated server, and jamp are running, hit ctrl+alt+delete, than go to task manager
2. go to processes, and right click on jk2ded(your server program) than click on set priority, than click either high, or realtime. which ever one you think you need.
3. click on jk2mp(the mp game) and right click it, set priority to medium or low, which ever one you need.
This makes a significant difference on CPU usuage, and helps your server to be more smooth (that is if you are playing on the same computer that hosts)
Im so glad I found that out, I though I would share it.