How to speed up X over SSH on a slow network connection?

You will probably see the most major benefits by using compression using the -C option. You can also enable it in your sshd_config by using the following line:

Compression yes

If you have the option, try and test NX (although it is a pain to get going) as it will feel far faster than tunnelling X over ssh...

It's been a while since I tried it, but DXPC (Differential X Protocol Compression) used to make X11 over dial-up PPP work noticeably faster. Perhaps the SSH compression option will work better for you, but this compression is specific to X11 and just might work faster.