To be honest I have never heard of ffprobe. As for that setting no it's not checked. As for the install it's Arch so "sudo pacman -S ffmpeg".
Dang it ... would have hoped it could have been something like that.
ffprobe gets the meta information of media files, like the different streams and their formats and such.
ffmpeg is used for transcoding/editing/etc for the video and the meta data (like embedded titles etc).
Both very powerful tools! But I can imagine most people do not even know these exist even though tons of video related applications use it.
So ffprobe is typically used to detect resolution and codecs and such for each included stream.
RMTV uses ffmpeg to inject meta data (title, description, etc) if users wish to use it.
I think (could be wrong) the use of ffprobe for detection, and ffmpeg for injecting meta data, is by default disabled.
Under Windows (I'm a Mac user) I noticed that the first show search, right after starting RMTV, is rather slow. Any following search is fast.
Under macOS there is a tiny delay with the first search as well, but that is just like one second or so and all following searches are fast. Faster than Windows.
Under Linux I'm not observing any delay, not even with the initial search.
Tried Qt5 and GTK2 (under Manjaro 25.x) and both are instant when I drop a dummy file on it.
Maybe try that and see how fast that is? (just do "touch dummyfile.mkv" in Terminal to create an empty file and then drop that file on RMTV)