I have experienced this issue. I have a website where the MediaRecorder is (could be) invoked multiple times.

IT IS NOTED THAT THIS DELAY ONLY OCCURS THE FIRST TIME I ATTEMPT TO RECORD.

Subsequent recordings run/record as expected.

Since I have not seen any recommendations/answers, my first thought is to run a throwaway recording before attempting a real/used recording.

This is definitely not a great solution, but I believe it could be a workable temporary workaround... I'll let you all know how it works out.