I've got some ideas for some sweeping changes that will be made if you observe that replacing the v1.15a DLLs with the v1.14a ones resolves the slowness issues. Some other people are having serious problems with v1.15a as well: crashing when loading images, etc.. Going through the code with a fine-toothed-comb comparing everything I changed between v1.14a and v1.15a I just don't see anything that would cause the severe slowdown that you're seeing, nothing was changed that I would imagine would affect it like you're seeing - except for updating the SDL DLL files to the latest versions.
It's still spawning the same number of threads for doing background work and allocating even less memory than before - but we're seeing the slowdown globally affecting everything. What should take a fraction of a millisecond is taking multiple milliseconds, which is pretty crazy. I can only imagine it has something to do with the SDL DLLs and how they're spawning the window. I'm thinking I'm just going to strip all those dependencies out and hand-write the Win32 code to do everything super raw, and then use an alternative image loading library that some people I know are suggesting. It's a bunch of work I wasn't planning on doing, but at least it would eliminate the black boxes.