Revisit performance implications of commit 1eed8e8

[heuermh]

Commit message for commit 1eed8e8 states

On a single node with 20 workers, BaseRecalibration time is reduced from 5.8 min to 1.7 min.

whereas now downstream we are seeing the opposite

Running the reads pipeline on latest master on GCP on an exome shows a major performance regression. At the end of last year this took 40 minutes for the whole job, now using master BQSR is taking 1.4 hours, and HC is taking well over an hour (it hasn't finished yet).
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I tried reverting 1eed8e8 and the performance was back to normal!