Good observations! Yes LW does take longer to settle down if you have large or many incomplete files. It also scans your shared folders. I'm not sure about the re-scan process however. If you press the Refresh button within the Library, it will re-scan. The scanning of the files does take a large part of LW's energy & should be done sparingly if you have many/large incomplete files as well as many shared files.
I read some time back that apparently LW does have a max dwnld size capability. I don't know if that still applies or not. Nor can I remember what that size is. Perhaps 2 GB at a guess.

By the way, do you use a USB modem/router connection? I've read where some usb modems draw power from the comp & some actually utilise the comp's processor. This of course may affect your comp's performance in some cases.