I'll answer some of these questions now, and others later:

>I have tried again, without success, to recreate this, so here are 
>some more questions:
>
>- if you do world menu/help/vm statistics, have there been any full GC's?
>- how much free memory is Squeak showing?
>- just to double check, your VM is 2.9alpha5, right?

Yes.  I also tried it with 4 and 3: identical results.

>- when your test is running, it displays the event queue size on the 
>left. When you start to slow down, what do the numbers look like?

the stack is non-empty when things start to run slow, and usually 
stops on a small, but non-zero and non-one number.  If I type only a 
few events, it usually runs down to zero without the slowdown.  After 
all the events have run down to completion, the next event shows a 
zero queue.

This seems to result also when I strike several keys at once.

I'm going to test this across several machines at my house.