I have also observed this problem, not just in Evolution, but also in Firefox. In my case, the problem appears to be either (a) mail and web pages that contain images or videos that have problematic render engines on Linux, or (b) massive processor overload when web pages that have a large number of videos on them (such as Facebook) try to render them all simultaneously without limiting the resource utilization.
Because the problems are across more than one client application, I suspect that the problem is actually an inability to limit resource utilization by the desktop - in my case Gnome. It has been like this for quite a few Fedora releases now, and other than getting away from Gnome, I'm not sure what else to do. On an underpowered dual-core, these lockups are devastating, and sometimes even require an ACPI reboot to recover from.