If a transaction reads from multiple collections, those reads have to happen in series. This is a server limitation. The driver adds an additional requirement that all of these requests await the prior request’s response. This is inefficient: if I read from, for example, 10 collections, that’s 10 round-trips that have to be awaited. We’ve been trying to reduce latency’s effects on mongosync-driven migrations, so it’d be great not to have to wait so much.

It would be much more efficient to send all 10 reads—either `find` or `aggregate`—at once.