This allows requests to open and save/cache up to *max_workers* amount of TCP connections. In most situations it will still only save and re-use one TCP Connection since it always tries to re-use the connection if one is available.
However, in situations where downloads are from more than 10 Host/Port combinations (the default pool connections/maxsize) then this will improve download speeds.
The pool_maxsize value here isn't actually doing much. It should have also been applied to pool_connections. What we realistically needed was just pool_block to prevent opening too much connections (causing a warning). The default pool_connections=10 and pool_maxsize=10 is fine. The downloader doesn't currently use this value.