Interesting! I also worked in an incredibly stressful and busy industry before this and it surprised me at first how slow everything is.
In terms of communication, in the day to day I find email the absolute worst. It's usual for me to get 200+ emails a day, every working day on top of the endless meetings. If each of those emails takes a minute to read and answer and file, that is roughly three hours of my time a day just doing emails.
Broadly, we publish too many books is the root of it all!
I have to be quite strict with my email responses as loads of my authors email to chat, to procrastinate from writing and because they are a bit lonely working on their own. So even if I answer I get pondering emails back, or questions about elements of the publishing that take time to respond. Lots of authors are lovely but some are anxious and overthink, so publishing has become quite thoughtful in terms of written responses as staff know they might be read and re-read.
If it's a key author emailing I aim to respond in a day, but if I'm not actively working on a book I tend to go back within a few days.
For your friend who got the response two years later, I'd say his project was probably discussed at editorial, then dismissed, then it came back later and the editor just picked the email chain straight back up. The other thing with editors is that they don't always like to say a straight no so they just go a bit quiet.
Finally I get a weird lack of chasing emails - people sit and stew rather than reminding me of things, so my 'squeaky wheel' authors do get responses earlier to get them off my desk! Broadly, publishers are very under-staffed too.