I think you are right: the SLOWNESS is a killer. In fact, I think it is THE show-stopper flaw.
I recently succeeded in an 18-month-long effort to coordinate moving off of JIRA at work. We moved to clubhouse.io and it's been great. We were also looking closely at GitHub Issues + something like ZenHub for the missing project-management features — and that would have been great, too.
As soon as we moved off of JIRA, the amount of communication that started happening in the most relevant place — the bug tracker thread — increased by like... I don't know, something like 20x or 50x.
In our years on JIRA (the hosted plan) virtually nobody every discussed the details of the bugs on JIRA. We filed tickets there, and checked them off when done, and used it to review what had gotten done after each iteration — but nobody used it day-to-day. If discussion was to happen about a bug, it happened on Slack (which is horrible for ever finding it again, but it is fast).
The reason, I believe, is simply that JIRA is waaaayyy tooooo slow. Nobody wanted to use it day-to-day. You made a list of stories at the beginning of the iteration, and you checked it again at at the end of the iteration, to mark the ones you did done. I did this too.
Now, all the discussions directly about the bugs/features in the tracker happen directly on the tracker. Which, I recalled, was how it used to be several years ago before we moved to JIRA (from GitHub issues).
There is a lot of other stuff that is clunky about JIRA but I think we could have probably dealt with it, if it wasn't so slow that every interaction — even simple things like "find and open the bug I was working on most recently" or "see what is assigned to me this iteration" — felt like rm -rf node_modules && npm install and you somehow find yourself checking twitter before it even opens...
Speed matters!