"Whenever we will message on slack asking a question, they would respond 10-15 mins later almost 90% of the time."

I thought that was really good, I was expecting more like 30 - 60 min delay on average. That chat isn't real-time communication afterall, personally I have all notifications turned off.

Well for us it is very rare that your progress would be blocked by not having a question answered, that is why we (try to) have well-specified tasks and daily meetups in the morning.