DocumentSourceCursor may yield using

        bool cursorOk = pClientCursor->yieldSometimes(ClientCursor::WillNeed);

But it only does this after advancing, and in particular not before the first call to pCursor->current().

Diagnosed behavior: aggregation does not yield the read lock if its fist candidate matching document is not in memory.
Expected behavior: aggregation does yield the read lock in this case.