>What is going on? Why is it that simply adding a longer string causes the process to crawl?
I read in a perf discussion centering on when to use strings vs. StringBuilder that for a few operations working on strings is more performant than appending on stringbuilder. I think there was a limit on the number of additions optimized per line, although I am not sure.

OTOH, if you add larger strings, the memory reshuffling is greater and that can lead to non-linaer effects.

HTH

thomas