Mergeable checks get stuck

[fredchen306]

We installed the mergeable probot in an enterprise github organization, we have a repository used the mergeable probot heavily, and there are dozens of pull requests every day. Normally, the mergeable checks only take seconds to be done, but sometimes some take very long even more than 1 hour.
We did some investigation, it seemed if a pull request with 1000+ files changed created, in the meanwhile other pull requests(even with few files changes) got stuck most likely but not absolutely. We also saw "Rate limit" WARN but not sure hitting rate limit was really the root cause.
I want to ask:

1. If exceed the rate limit, will mergeable trigger a retry? so that the requests will not lost anyway?
2. I want to check at some point in time what the state of the mergeable server, if the workload is too heavy (we deploy it in a VM with docker), how should I do that?
3. Are there potential reasons that would cause this performance issue? We currently suspected exceeding the rate limit, but not sure how to confirm this.

Look forward to your reply.
Thanks a lot!