Quite frequently, usually when a folder with sub-folders has been open quite some time, it takes a very long time - tens of seconds - to close the top-level folder. It may be relevant that these folders are empty, but the subfolders are not. There are over 250K messages in one tree, and over 10K in another.
System is Windows 7 x64 Ultimate (clean install), Pocomail 4.8.0.4400. Other apps running are typically Google Chrome, Office 2008, and the usual range of applets. Antivirus is Microsoft's plus Malware Bytes. The PC has 4 GB of RAM with 3 GB currently used. I keep my PC on - and Pocomail running - 24/7 and has been up for 10 days, but I've seen the behaviour after Pocomail has been running for less than a day. Pocomail is currently using 563 MB.
I cannot reliably recreate the problem.