Found the answer, and it turned out not to be jqgrid that was the bad guy, but the server-side Session store! I used Session as a cache for the grid data, because I needed the filtered data for other purposes than the grid, and wanted to avoid redundant trips to the DB. Once I wrote something to the Session object, the server took a hit and started handling all incoming requests slower (often several seconds!). I have later learned that using the Session object for caching is not advised in most cases, but I still don't know why it would cause nasty side-effects like this. If someone would enlighten me, that would be great! It cannot be an issue of taking up alot of RAM on the server, because the performance dropped just by writing

Since I actually needed the data to be cached in a session scope, I solved the problem by instead using the HttpContext.Cache and a session-specific key.