The FCC has published a report (PDF) on the performance of wired broadband in the US. In addition to comparing sustained upload and download speeds to what's being advertised by various ISPs, the report looks at shorter burst transfers and UDP latency. Sustained data rates are provided in two flavors: a 24-hour average and a prime-time period from 7-11 PM.
As one might expect, performance can slow considerably during peak hours. Some ISPs offer more consistent transfer rates throughout the day than others, though. Cablevision looks like the worst of the lot, with download speeds dropping to as low as 50% of the advertised rate. Overall, cable services come closer to their advertised performance levels than DSL. But they're no match for Verizon's fiber service, which delivers consistent performance regardless of the time of day. In fact, the fiber service's average transfer rates were measured to be faster than Verizon's claims.
Only a few ISPs delivered better-than-advertised download speeds, almost all of which were during off-peak hours. Interestingly, most of the services tested offer higher upload speeds than what's been promised to consumers. Although region-specific data isn't available in the report, it's worth a closer look if you're in the market for a new ISP or wondering how your current provider stacks up against the competition.