@L--U--P--U--S ummm, ahh, Windows, I missed that, yeah well the windows downloads are gtk2 up to 1.37, the intention is to switch to gtk3 before 1.38 release, but it depends on peoples time availability which is unknown in these "interesting" times. So you would have to build it yourself at the moment.
hmmm, 15 files of a few hundred lines taking 11 secs, on Linux I'm testing with approx 120 files taking about 6 secs, some of them being several thousands of lines (basically Geany and Scintilla src directories) and not seeing any difference between 1.36 and 1.37, but @andy5995 is seeing it on Linux. I'm not on a blindingly fast machine, its 9 years old, but it is from SSD, I wonder if there is something about some disks? And windows? I'm not aware of any big changes to how Geany reads files between 1.36 and 1.37. If I get time I will try loading the files off spinny disk.