After upgrading my mongodb driver from 2.2.33 to 3.0.1, the time to connect to the server, even to the local machine, increased from a few milliseconds to over a second.

I tested and verified that this is the line causing the slow down.
const client = await MongoClient.connect('mongodb://localhost:27017');
Other operations are as fast as on 2.2.33.

I tested the same issue on a mac running High Sierra 10.13.2, no issue there. This looks like a Windows issue.