Sorry we've not reached your post sooner and I would have responded to your PM more quickly if I'd been in work last week.
I wouldn't be overly concerned about the change in downstream power levels. Rule of thumb is the closer to zero the better, so those levels are optimal (with + or - either way by a few dBmV's being absolutely fine).
There has though been some upgrade work taking place on your CMTS which may have been causing some disruption to performance. However, that is unlikely to cause your GUI access to run slowly, unless unrelated Internet pages were also affected.
If your speed is still adversely impacted I would like to know whether or not you're connected via Ethernet or wireless and would also like to know if removing your own wireless router (and hard wiring a pc directly to the Hub) makes any noted improvement?