Summary:
Decoding large actions is a plausible culprit for hogging CPU time for
certain documents. To begin with, log the time taken for this operation,
so that we can tell if it's a problem in practice.
Test Plan: Should not affect any current behaviors
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D2989
