I don't use atomic myself. But I find that enabling the back-end thread slows down the UI threads. I've been struggling to create a simple program to illustrate this.
In fact, wrapping parts of my UI threads in atomic actually makes them more responsive[1]. I think it's because instead of getting 200 thread switches with their associated overheads, a single thread runs to 'completion'.
[1] You're right that throughput may go down, though I try to eliminate IO in atomic blocks. But latency is more of a concern to me than not wasting CPU. It's not the end of the world if new data takes a few seconds longer to make it to the index, but a few seconds in the front-end can make or break the user experience.
"vector-set-performance-stats!" in http://docs.plt-scheme.org/reference/runtime.html returns the number of thread context switches that have been performed, so you might try calling that at the beginning and at the end of an UI operation so you can see how many thread switches happened in between.
