I am getting this too. Twitter's character counting (java?) script is probably the problem.
There is no Tools>User Agent option. Since Twitter is so popular these days it is a shame that it is real tricky to use with K-Meleon.
Sorry cannot help because I have no twitter account. In general it looks like most of such slow scripts work fine in the next KM version, KM1.6alpha, so there's hope ;-)
But wondering why don't you have the user agents menu?? It's in the default profiles, so either you have disabled it, or perhaps just overlooked it, or are using a very special KM version...? I did check in KM1.6alpha, and see it there too.