You might be running into this bug Slow StringTermsAggregatorFromFilters · Issue #76104 · elastic/elasticsearch (github.com) in 7.13.x, which was fixed in 7.14.0.
You can see if this is the case, by using the issues provided workaround, setting the following cluster setting:
"search.aggs.rewrite_to_filter_by_filter": false
I ran into a fairly similar issue not too long ago with a somewhat simple agg, and this turned out to be the issue.
But as mentioned previously, being able to see the query and agg you're actually running would be helpful here.