It can happen that kmail loads a folder for a very long time, and within that time no other operations are possible.
This happens due to an ItemFetchJob blocking the complete session.
The ItemFetchJob takes so long, because requested parts are not locally available (cacheOnly() is false because we need on-demand fetching),
and the ItemRetriever then ends up requesting each message in the folder (typically several thousand), one by one. This naturally takes forever,
and the fetchjob remains blocked during the whole time.