Sync gateway logging "call to Do" actions that I've never seen before

We haven’t changed our log levels for sync gateway, but I started seeing logs like this in our sync gateway logs recently:

_level=INFO _msg=go-couchbase: call to Do("_sync:rev:DOC_ID:36:438-74df49f3b7e8a41d84e1f2580d1c51fe") in*Bucket).GetsRaw took 209.090131ms 

_time=2019-09-09T17:13:57.959+00:00 _level=INFO _msg=go-couchbase: call to Do("DOC_ID") in*Bucket).casNext took 522.571127ms 

Why would sync gateway start logging these actions if we haven’t changed any configurations?
Sync Gateway Community 1.3.1

These are only logged if the operation takes longer than the specified SlowServerCallWarningThreshold, so it’s likely that your Couchbase Server is performing slower and taking you over the threshold for these logs to start appearing.

That makes sense, thanks! We’ve been having server crashes about once a month lately due to load, most likely CPU load since right before the crash sync gateway is doing something that causes it to take 300% CPU (we have 4 cores) for a while.