What do you mean? With the current internals, a 300 posts HN page would weigh ~500kb; different ones will hardly be more compact. Where is the «struggle»?
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.
HN has been known to fail in the past with heavy or high velocity threads to the point that dang has asked people to log off en masse to reduce server load. That shouldn't happen for a simple text forum.