Firefox slow over ssh -X (again)

As pointed out in a comment to the question this was about gfx.xrender.enabled. The answer can be found in the marked dupe, but it is easy to miss in all the noise talking about ssh and the X11 protocol. Direct link to answer: https://unix.stackexchange.com/a/557920/66307 My problem started happening about a month ago. I'm currently on Firefox 89.0.1. Up until this point, Firefox ran just fine over ssh -X. I'm running it from an Ubuntu machine (server) to an Ubuntu VM (client) and a different Ubuntu machine (server) to Windows 10 (client), both showing the same symptoms. It's not my hardware setup or -X as all machines are wired to a local gigabit switch and other applications (including Thunderbird) run just fine. They are a little slower than if I was running them on their own desktop, but perfectly usable. After it happened, Firefox is just .... slow. I'm using it with --no-remote and the actual window for Firefox appears in under a second. It then hangs there for about 2 minutes until the tabs become usable. The output in my ssh shell reads: $ firefox --no-remote & libGL error: No matching fbConfigs or visuals found libGL error: failed to load driver: swrast (firefox:757486): GLib-GIO-CRITICAL **: 10:39:54.241: g_dbus_proxy_new: assertion 'G_IS_DBUS_CONNECTION (connection)' failed (firefox:757486): GLib-GIO-CRITICAL **: 10:39:54.241: g_dbus_proxy_new: assertion 'G_IS_DBUS_CONNECTION (connection)' failed (firefox:757486): GLib-GIO-CRITICAL **: 10:39:54.241: g_dbus_proxy_new: assertion 'G_IS_DBUS_CONNECTION (connection)' failed libGL error: No matching fbConfigs or visuals found libGL error: failed to load driver: swrast libGL error: No matching fbConfigs or visuals found libGL error: failed to load driver: swrast but that doesn't look too different to what I had before. Can anyone please suggest anything that might speed it up? I'm not really looking for answers telling me that ssh -X is slow (it is, but it's acceptable to me on all other applications) or things to do with compression or cypher suites. My link is fine, except for Firefox.