Just before you send the Server RPC to change walkspeed on the server, set the walkspeed on the client, then the client doesn’t have to wait for server round trip to replicate walk speed back to client.
On the other hand, this is a Character Pawn we’re talking about. It might be replicating and predicting movement speed already without RPCs, and adding RPCs might double the effect, thereby causing jitter. I don’t know without looking in the character pawn code.
Try setting walk speed just on the Client with NO RPCs and see if that works better. Character Pawns have a lot of hidden networking replication and prediction stuff already built in to them, that works better than RPCs alone.