Are you running SlickEdit on your home system or accessing a remote instance of SlickEdit running on your development machine?
If you are running SlickEdit at home and accessing remote source, it is best to still have the SlickEdit workspace (.vpw), project (.vpj), and tag (.vtg) files stored locally. You should also be sure that you have the config file stored locally.These are referenced fairly often by SlickEdit and the additional latency can be a problem.
What specific performance issues are you seeing? Each will be the result of different latencies. For example, if it takes a long time to open a source file, it could be that the network latency of the remote source is very high. If it is that completions are very slow, then having the tag files local should fix that.
There is some excellent material in the User Guide related to this. From the main menu, select Help > Contents and check out Introduction > Performance Tuning > File Locations.
Also, you should close the Projects tab and be careful about opening it. It tends to crawl through the project structure. With a large remote codebase, that can cause performance issues. You should also avoid using wildcard projects with large, remote codebases. This would show up as a problem if it takes SlickEdit a long time to wake up after switching applications.
Let us know if this helps. If you're still having issues we can try some more things.