Remmina uses freerdp as its RDP library.

I found it to be somewhat faster when comparing between the packages my distro had, but YMMV.

What did make a _noticeable_ difference was to download the FreeRDP sources and compile everything statically. That blew Remmina out of the water. I did not do the same for Remmina though, so the comparison is not fair.

Remmina is a just GUI frontend. You can choose a RDP backend between FreeRDP and RDesktop.