I don't often have trouble with slack being slow, or buggy. Been using it like 9 years myself. Interesting you're comparing slack to icq. Are you referring to a current version of icq, or the one that existed in the early 2000s?

I am not sure I understand comparing an app designed to do video/audio chat seamlessly, threaded conversations, channels, filesharing, plus has dozens of subtle nice features that make for a rich experience and a... Chat app, that worked fine for sending plaintext messages but didn't really do anything else.