Deleting the incompletes has an effect on LW. It means it loads faster, settles faster, less VM drag due to the incompletes trying to connect on their own & LW handling the overall load. It's very much related to OSX's Java & its inefficiencies. I have a lot of incompletes & my LW runs slowly. Stief suggests someone on OSX shouldn't share too many files (above 2,000). I took out most of my shares so I had less than 100, & still my LW has some drag issues. It's due to the incompletes. So I am now more confident to have replaced my shares (though I keep them not much above 2,000 anyway b/c it does also probably affect things.) So why don't "i" delete my incompletes? Well I seem to still occasionally hook onto one of those that's been sitting around for a year or longer. Most of the files I seek are not common files. If there's a chance I might get these rare collector's files then ... And I do delete the odd few here & there anyway.

Defragmentors for OSX: I use Norton Utilities -> Speed Disk (but only very occasionally - less than in OS 9.) However I generally only use it on my other partitions & HDD's, rarely on the OSX partition (system drive.) The reason is I heard fragmentation is much less an issue in OSX due to the way OSX handles files. Perhaps if you're doing video work it would make a difference but I do my video work on designated partitions/separate HDD. BTW iDefrag is another OSX defragger but I tried it out & didn't like it. Incredibly slow & I somehow don't trust it.

View incoming: I always have this disabled since I feel it's only adding extra consumption of resources to LW & Java. M.O.O.

Speed test: LW should be turned off at this time so you can get a good idea of the proper speeds your connection is capable of. More tests here (choose nearest location): http://www.t1shopper.com/tools/speedtest/
But if they quote speeds in kbps (kilo bits/sec) then you'll need to divide that by 8 to give you KB/s (Kilo Bytes/sec) which is what LW uses.
After that you can try downlding a magnet link from here http://www.magnetmix.com/video.shtml to see what speeds you get thru LW.