The slow file loading might be the same reason as in #2649.
The interface freezing sounds like the initial problem in #2837. I have no idea what this might be (except blaming GTK).
I'm not sure about #2837, but #2649 sounds about right. Curiously this user mentions that version 1.37.0 slowed down file loading dramatically on Windows: https://github.com/geany/geany/issues/2649#issuecomment-724718783
I also noticed today that although almost everything remains frozen, left clicking the documents on the left actually changes the file name in the window title, but nothing else, including the text editor which contains the same text from the file focused after startup (based on last focused file from previous session I believe).
I also just want to highlight the last part of my post because I at least found it very curious that although file loading is slow either way, the interface is not frozen if the file automatically focused is small enough.
@poikilos thanks for confirming, not many of the usual Geany contributors use it on Windows and as @Mikkel-Skofelt said, it works just fine on Linux.
Everybody please confirm by posting the top few lines of Help->Debug Messages that you are using 1.37.1 which is the latest Geany for windows.
Sorry about that, I forgot to check and mention this, I do run geany v1.37.1.