Who said anything about CPU? Slack's going to make DNS requests, multiple TCP requests most likely. That's network IO, multiple orders of magnitude slower than reading from disk - which is (at most, it may just read small file from disk and unpack in memory - I don't know) what TempleOS does.
Slack's app has to display images, render scrolling, respond to network events, store and load application state like your preferences and theme, etc. Assuming it does these things - I use the web client.
Slack also has to support multiple operating systems, so it may make technology choices around that.
It's really not hard at all to see why Slack would be much slower than an OS that basically does nothing to boot up.
It's not hard at all to see why Windows, which supports tons of hardware, would be larger on disk and slower to unpack as it boots - lots of drivers and services need to start up to support all of the things that make it not TempleOS.