I've switched the ajax handler to a the custom ajax handler on the “Compatibility Options” submenu.
WordPress is not famous for it's speed in general, and unfortunately each ajax request has to wait till every plugin and core function is loaded which takes time, usually around 1 second. Only then it starts to actually work on the ajax request. By turning on the custom ajax handler, some of the loading time is decreased, so you should see faster results now. (1-2 second difference)
Another thing that takes some time is the thumbnail generation. If the thumbnail is generated for the result for the first time, it take a bit more time. But the second time it's cached, so no generation needed, thus the search is faster. If you try one search term two or more times, the second time it will be much faster – because the images are now generated.
Also, if you want I can install the upcoming version for beta testing. (not completely finished, but working) It has some other improvements as well, which should affect the search speed greatly. I'm releasing it very soon, and some feedback would be nice. Let me know, and I will install it via the plugin manager.Best,
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