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Indeed, the Windows app is horrid. One of our older workstations is a quad core i5 with 8GB, nothing special but no slouch either, and the Slack app completely chokes that machine. The employee has to run Slack on her iPhone to be able to get regular work done throughout the day. On our newer workstations it's not quite as bad but it's still noticeable. Our oldest in-service machine is a Core i7 laptop I use when I'm…
Out of interest, what's your concern with Skype? After a hiatus of ten years, I'm using it currently for a customer, and, maybe apart from cheesy emoticons and space inefficiency, so far it has worked well. Am I the only one to like a native (Linux) desktop client with notification integration etc. more than a bloated web/Electron app?
Sometimes when trying to log in it would say that the (saved) password was incorrect, the user would go change it in their Microsoft account and it still wouldn't take it, then a few hours later it was working fine again.
Slack also has much better search, and their support for attachments is dead-simple. Sending a file on Skype was always a chore that felt like it was 1999 all over again, but on Slack it's literally drag, drop, and collaborate.
As I said before, Slack wasn't my first choice but it's a good platform for what it is. I don't like that our conversations and files are stored on their servers, but I doubt anyone at the company is sifting through our stuff looking for dirt or valuable info on a small business with 10 employees.