Note: I did not shut down dashboard because of reported memory usage, nothing was obvious from activity-monitor either, ie not listed separately as a memory-hog.

However, once I did shut it down (not just remove all the widgets, I had like 10 running, but actually prevent it from starting) my machine was finally back to normal.

I'd be curious to see if anyone here who does experience the memory problems can replicate this.

Addition: since shutting it down, the reported memory usage for the kernel task has dropped a lot, does that make sense?