The performance isn’t the best, when compared to RDP. Protocols like RDP can take shortcuts by treating it more as an image then a fully remote rendering, like image compression techniques. For tunneling an X-window over SSH, the compression option you have is with the connection, itself. Using say, ssh -CX or ssh -CY allows ssh to compress the connection for better performance, but that compression able to do lossy image compression. The killer will be latency. If it’s over a highly latent connection, like a WAN or satellite, it will be rough. But on a wired lan, or decent local wifi, it’s pretty all right.