investigate slow uffizi read performances from the object store

In the context of #7 (closed) we started copying uffizi → banco. The estimated ETA was ~50 days for ~50 TB, i.e., 1TB/day which is definitely too slow.

Oddly enough, the bottleneck seems to be neither the network, nor the writes on banco, but rather the reads (!) on uffizi.

We need to figure out why it is so.

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Migrated from T274 (view on Phabricator)