I have the same problem. But for me, it only happens on very large torrents, and when I have downloaded a lot of it... Downloading multiple 30-60GB files is no problem. But I have one torrent jet to finish, and it is almost 110GB (one single file). Actually it contained multiple .rar files. All of them together is about 133GB, but I removed the other ones when they finished, to see if it would help. But it didn't...
If I remove the torrent, then qBittorrent works as it should again.
Running any number of files and or sizes have never slowed down qBittorrent's closing before. Like I said, it only happened when I had a certain amount of the file on my hard drive... Unfortunately, I don't know where the limit occurs. I am running Windows 7 64-bit Home Premium, quad core CPU, 8GB of RAM and 3TB Western Digital RED drive. And like others have mentioned, when it does happen, it's not possible to kill the process. It will disappear, when closing, but still run in the background. When I close Windows, it will complain that qBittorrent is still running and that I should wait for it to close. I have waited for 1h, and it never does... closing qBittorrent without this torrent, takes seconds no mater how many or big (at least up to 60GB) the other torrents are.
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Edit:
Forgot to mention that I am using the latest version (4.1.1) of qBittorrent.