https://medium.com/@matt.at.ably/wheres-all-my-cpu-and-memory-gone-the-answer-slack-9e5c39207cab :
TLDR; If you care about battery life or availability of your finite CPU and memory on your computer, then you probably won’t want to use Slack desktop with more than one or two accounts. Slack resource usage increases linearly as you add more accounts, and it quickly adds up.

28 July update: One of the Slack developers got in touch and recommended I try out their latest beta version in which the CPU issue appears to have been fixed. See below.