Thanks for your reply! I did some more testing last night, and I was able to gradually open a bazillion programs at once, jacking up ram usage to 40% with no slowdown. I still don't know what's causing this, but I'm going to try to systematically work through some boot options.
Would it be worth trying an older version of DSL? I ran into this with puppy; some of the newer versions didn't work when updated.
Thanks again,
Steve