I had that happen once (or like fifteen times) when I was researching how the "Export to Static HTML" function worked:
http://wiki.modxcms.com/index.php/Export_static_html
In my situation, the manager was crunching through every page of my site, and sometimes it flat out kacked. I had to restart Apache.
So, even though that may not be your situation, I can tell you that even WITHOUT something like that, when I restart httpd (apache) and mysqld on my servers, I notice a boost in speed. This is true even on my dev server where there aren’t any other people using it... I think this might just be the result of orphaned processes being cleaned up that build during normal use... or maybe I have something misconfigured, so my speed sorta leaks and leaks and my site gets slower and slower.
Hope this is relevant.