This has happened to me throughout all the versions of SWT Designer I have used (going back to 1.1 I think). Basically, as I use the Designer, and switch to source, then back to design, then to source... you know, typical usage, things just start slowing down. Not just the Design editor, but also the Source Editor becomes very sluggish (scrolling, code completion, etc).
Sometime closing the offending editor and reopening it speeds things-up again. But this morning, while giving a workshop on plug-in development, I had to close Eclipse entirely to regain the speed I needed. A little embarrassing.
I don't believe I am receving exceptions during this slowdown. It has all the symptoms of a memory leak.
But, who am I to tell Instantiations the cause?

-WinXP
->2 GHz
-1GB RAM
-Eclipse 2.1.2
-SWT Designer 1.2.3GA
Please, others with this issue share your thoughts to help us find the cause.
Thanks