With Jetpack installed & activated, deleting spam comments times out after only deleting a small number in 30 seconds. Deactivating the plugin resolves this. Since there are several thousand spam comments I needed to delete, doing them at a rate of 5-7 seconds each was going to be a problem.
I've also seen problems with the automatically-on Contact Form portion - I haven't added those forms anywhere on my site, and yet I have 6700 spam "Feedbacks" with absolutely no indication on the Dashboard or navigation menu that they existed. I only found them because I was wondering why my wp_postmeta table suddenly had 69,000 rows and was over 40MB when I have under 50 public posts.