I've used Hey daily since it launched on desktop and mobile and I can't see it being slow in any way. I guess you could count that it shows a loading symbol for a file to appear (on the first load only).. but that would be excessive nitpicking.
It never performed slowly for me. It ended up having none of the features I wanted, but that’s a different concern altogether. Definitely rode the hype train and feel dumb about it now haha