I'm having similar data issues both in downtown SF (I'm on Sutter between New Montgomery and Market) as well as near my home in San Carlos. Just absolutely AWFUL LTE bandwidth. Almost unusable at the office and completely unusable in San Carlos (so much so that I filed a support ticket with the Network Operations center over 2 weeks ago and have yet to get an update on the issue).
Union Square - terrible!! Dropped calles and my iPhone shows LTE while it take FOREVER for anything to load. They really are taxed in SF though happy to take your money while delivering a sub-par network in SF. Sad. Maybe time to check out TMobile.