OSX Java is very much to blame here. It's not as efficient as the other Java's. Mac do their own implementations of Java which is the pity. I noticed improvements after upgrading to Tiger & updating Java to 1.5. But that was on my system. Other people didn't notice such a big improvement. But still 1.5 has problems but not quite as bad as 1.4.x

Reducing shares but more importantly keeping incomplete files to a small number makes a major difference to how LW behaves. Having over 2,000 shares is risky on OSX with this issue.