Plus I really dislike those slow animations and really can't stand them, but that's just preference I guess.
I think this is one of those sites that was designed and tested on the developers machine and looked good on the browser they used, but wasn't tested anywhere else. But given it's probably just a hobby project, I can't really blame them for that at all.
The design is pretty good, but you can't get away with `position: absolute` these days unless you need it to set something steady on the screen.
`overflow: hidden` on an open full-size modal is also not a good practice, I agree.
Is 300ms too slow for you? I've always considered 250ms to be a generally-agreeable animation time.