We have a very large migration project on at the moment and the nodes take more than a day to run a full migration.  We're at the point where we really don't want to rerun the whole thing and we're seeing some things that need updating. The quick solution was to fix these in the process plugins for the actual migration and then create a quick 'fix' migration that just does a simple SQL update on a field table.  The updated process plugin to fetch the source value was also a very lightweight SQL query that didn't require any entities or objects to be loaded.

The NoOp destination plugin was needed because the NullDestination plugin appears to not work on purpose. This left a situation where you could never have a migration without a destination.