The read raw operation is designed to be an optimized, low-latency file read operation. A server may choose to not support it, however, and Samba makes support for read raw optional, with it being enabled by default.
In some cases clients do not handle read raw very well and actually get lower performance using it than they get using the conventional read operations.
So you might like to try read raw = no and see what happens on your network. It might lower, raise or not effect your performance. Only testing can really tell.