It's possible... I walked a couple blocks to a community event at the park with my wife and kids to watch fireworks on July 4th. It was crowded at the event... and speeds were at .50Mbps... but everyone around me had slow speeds. Even my Verizon portable mifi was slow. Walked back to my house after the firework show, and wasn't throttled anymore. I expected congestion at the event... so I can understand the slow speeds. That was only time and location I had issues with slow speeds.
That's not throttling.
That's an overcrowded cell tower with thousands of users connected at once bringing it's performance down to its knees.
That happens at stadiums, events and such occasions where overcrowded places cause towers to get slammed like that.