I reworked the client file manager's locks to be more fine and sophisticated. When you have several importers working in the background, and particularly importers handling large files, the client will delay access to your files and thumbnails far less, and generally not at all. I have been thinking about the specific change I intended to make here for a while, but this stuff can be tricky and I wanted to think and be careful how I did it. All tests so far are proving good, but let me know if you run into any trouble.