I'm accessing Thunderbird on my desktop (Opensuse 15.1) over ssh (over wifi) from a laptop running mangeia 7.1 and its painfully slow. Firefox is almost as bad.

Raspbian on the laptop is even worse!

Idle3, Filezilla, Leafpad and Shotwell run as well as directly on my desktop, as far as I can tell.
The command I use is "ssh <ip address> -X -l <username>
The desktop has no firewall. Wifi speeds are fine.
Any ideas? I'm going to read through the man pages for ssh but i'm no expert.
Does ssh limit transmission ? Can it be changed ?
Its a real pain because I'm isolating away from my desktop, I can get to it but it requires a bit of organisation.
Any help welcomed

ssh x forwarding will be slow with "heavy" GUI applications. You can try using different ciphers and/or compression but I doubt that will make much of a difference. Using a remote desktop application like nomachine NX would be much faster since it uses differential x protocol compression.

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Two decades ago X graphics was very efficient, and fast over a network. The "intelligence" was at the Xserver/Xdisplay (that was directly connected to the graphics hardware).
But new features like themes and half transparent windows would require a redesign of the Xserver and extensions to the X protocol.
Therefore the "intelligence" was moved to the X client. E.g. a 20 byte text is no longer sent to the Xserver for rendering and displaying, but is rendered on the X client and sent as a picture (say 16 kb big, even bigger with a high resolution).

Thunderbird is definitely one of the heavier applications, probably even more so than Firefox (although UI-wise they're essentially the same thing).

Many thanks for all your replies.
Ive finally got VNC access to my desktop but only after installing Xubuntu 19.10 on a free partition on the laptop and using Remmina.
I'd tried from mangeia 7.1 but all the VC viewers kept giving me "Invalid Display Settings". Ive fiddled and searched for days with no success so this is a HUGE THANK YOU to the developers of Xubuntu and Remmina.