Not sure about 300 comments but a post with 5300 comments takes about 10 seconds to load:
https://news.ycombinator.com/item?id=43208973
In March this year, HN changed pagination behavior. Previously, one needed to paginate through pages to read more than X comments. Around March, they now serve all comments at once.
A post having over a thousand comments is extremely rare so not a big deal.
That page took well under 2 seconds to load on both desktop and mobile for me (wall clock).
For me it consistently takes 8-10 seconds. Back in March, I reported it to dang too.