LW 5 does not handle large quantities of shares or large amounts of incomplete files very well. In my experience the two most guilty media types are music & images. Only files that show in your P2P network window are files that are being shared, unless you also are using the 'Share with Friends' option, which I haven't tried, but guess such files would show in that window.
BTW in LW 5, when you Cancel a download file from the Download window, it also deletes the incomplete file which is very different to LW 4.

Old LW preferences folder 'might' also slow your LW 5 down.

Personally, I do also prefer using LW 4. LW 5 is still missing many of LW 4's tools, some of which will not return but some will at a future time so I've been informed.