There has been no legitimate evidence that USO has been slowed down. AO has always been slow, hasn't changed.
They slowed it in 2003, said so publically. 2001 also maybe? Memory struggling there.
I believe it was speeded up a tiny touch in 2008, and the court pace has remained constant since. It's pretty closely monitored, easy for them to measure. Of course, you do get variance court to court, the more play there is on hard court the faster it gets. Weather is important though, and obviously it's not that warm atm.
The balls: Wilson US Open, I think are marginally larger than in the late 90s too, but that's harder to verify. Got lots of measurements since 97, but quite what method you use is open to debate. They're still one of the quickest of current pro level balls.