Well, i don't think speed was the issue, but when I went into the vSphere Web Client I saw more specific errors. In particular, the esx host failed a task of entering maintenance mode, and that appeared to be caused by a failure to migrate a vm ( the vShield Manager actually ) that was on that host. And it seems that the failure was perhaps related to storage being too full to handle this migration.
I'll post more once I sort it all out. Hoopefully, I'm on the right track.