I don't know if it's exclusively an issue with FO:London, but I remember having the same issue with Vanilla FO4, and turns out the loading is weirdly linked with vsync or something.
I'm not at home so I can't check which mod I had installed, but this one seems like it might be it, which basically unlocks vsync specifically on the loading screens. I remember going from several-minute loading times to just a few seconds everywhere.
Load Accelerator
Reduce the loading time by controlling vsync(fps) and CPU affinity at loading screen.This is expected to shorten the loading time under the SSD environment.(Under HDD environment does not
www.nexusmods.com