I'm remote from time to time. I really can't think of how the synchronous nature of Slack helps anything. If you need real-time chat, it's faster and more productive to have a phone/video call. If it's a long chat with lots of detail, email or issues? If you are spending > 10 seconds writing something to someone on Slack, you are already wasting time - if it's important, it needs to live elsewhere. It if it just "chat", then talking is faster.