How can I reduce time spent acquiring the mongodb schema lock?
This is a general architectural problem with the WiredTiger storage engine.
It is now being discussed here: https://jira.mongodb.org/browse/WT-5479

Long story short, your number of open files is too high.
If you can, consider removing unnecessary indexes, e.g. indexes on small collections.
You could also explore the new wildcard indexes introduced in MongoDB 4.2