why did Lotus desktop download emails only every 15 (?) minutes?

I remember people phoning to say "I"ve sent you an email, press the manual refresh button to get it faster".

up to this day I ask if companies use Lotus and those who remember just shake their heads

All email was like this at one time. In the 90s we were told to expect emails to take a few hours.

If you want immediate response use IRC or actually phone someone.

Think about it in context. Polling constantly would consume a non-trivial amount of both bandwidth and server resources. 15 second intervals would require 60 times as many very expensive servers to handle a certain number of users as 15 minutes would.

And in the 1990s, people were just adopting email, and their existing context was mail that arrived daily. Other than IRC, nobody had ever experienced any instant instantaneous form of communication other than phone calls, so nobody really felt like they were missing anything if they didn’t get notified about an email for 15 minutes.

Today by contrast, I feel like 3/4 of the time I go into my email client it’s to get some login code sent by some stupid website, so from this perspective 15 minutes sounds like absolute torture.

I still manually check for mail. I don't want distracting notifications popping up at random.

This seemed a little hard to believe but it sounded like it was more of a replication where it was downloading things into an offline mode where you then write your emails and send them back again period. Other email clients might do this as well if you're running it on your desktop.