I recently upgraded to 3.5.1, and that appears to have been the change, though maybe there is some way some other plugin is wreaking havoc with the system?
I have a custom field that had an image selector in it. When I click on the button to bring up the dialog to select the image, it now takes 100% of one of the CPUs almost a minute before the dialog appears. We do have a lot of images (99 pages worth). I originally blamed disk access time, but after looking into that the disk is pretty idle, and the CPU is very heavily used.
My PHP processes used to have a 30 second timeout, and that wasn't ever hit, but now I've had to up the limit to 60 seconds to get the thickbox_results() function to return with data.
I'm not thrilled about having to up the limit, on general paranoid principles, and obviously waiting a minute to open (and then another minute if I click on a different page within that dialog) is pretty annoying for a user.
Is there anything I can do to speed up that dialog? My images are organized in the date folders, so there aren't many files in any directory. 4791 files total.