If I look at my SAIX usage stats, I can clearly see that there's another IP on the account which definitely isn't mine. Thing is, it's been that way since I got the account and was even more obvious in the Isdsl portal when it was accessible.
Anyway, my point is, even though there appears to be another user "sharing" the account with me, up until recently performance was great, so I really couldn't complain.
It was only at the end of April that things started getting bad during peak hours and even off peak was slow before midnight. I made the assumption that a third user had been added to the account, and hence decided it was time for me to move on and submit my cancellation notice.
After examining the account session history carefully, I concluded that there's still only one additional user on the account. Hence, I can safely assume nothing has changed since I got the account.
So the only explanation which makes sense is that Openweb has implemented far stricter shaping policies across the board for certain accounts. Perhaps they're making room for new subscribers without purchasing additional capacity?