It can do at least three calculations per second.

How long is a piece of string?

Sure it can, but it depends on what software you run and how much load you put on those services.

Yes.

Xmpp, irc, email and websites are all pretty small in terms of resource usage. Where you’ll have problems are with heavy compute workloads like 3d rendering, AI, 3d gaming (sorry, no crysis), and crypto mining. They all can be done on a rpi but not done well.

Oh, it can run even more! Under one little condition - nobody uses them.

i run homeassistant, pihole, 2 projects of mine using NodeJS, nextcloud, a very simple static website and Nginx for some DNS customization like pointing nc.home to a specific port. My pi is a 4GB one and for now the used RAM is not even 1 GB. I run debian with no graphical interface