The main things that can slow it down:

1. Sharing too many files.
2. Having too many incomplete files.
3. Using the resume/find sources on too many files at one time.
4. Having Java version that's too early.
5. Using the media player whilst downlding.
6. Using another p2p sharing program at the same time.
7. Not having enough ram.
8. Not having lots of available HDD space in the event of virtual memory use.
9. Having a HDD that is highly fragmented. Particularly if it's over 75% full. Drives lose speed above this capacity.
10. Having too many old searches open for a long time.
11. Keeping the view incoming search messages open.

Some other people may add to this.