I'm getting subversion timeouts, even when trying to check out just one
branch of pkgsrc (e.g. HEAD_660).

This has been from my local box (fast comcast) and dreamhost shell
accounts. It hasn't happened from an MIT box I have access to; I'm guessing
because of lower latency.

Currently I'm doing an svnsync from the MIT box with hopes that moving that
to my local box and then doing incremental updates to that from my local
box via svnsync might take less time than updating via svn.

I'm not sure what the setup is on your side, but this is one suggestion I
found that might help:

Solving checkout problems with svn repositories

Potentially you can have problems when checking out a large svn repository,
if on sever side mod_dav is used. The SVN client might fail with the error
message "Could not read response body: Secure connection truncated" or
"Could not read response body: Connection reset by peer".

To fix this issue edit your subversion server file located at
"~/.subversion/servers" on linux or "cd %LOCALAPPDATA%\Subversion\servers"
on Windows.

Simply add extremely large timeout value

[global]
http-timeout = 6000