How to tell if a folding bike is faster than a boris bike Am considering purchasing a folding bike, but curious to know if there is any way to know (other than planned / timed test riding)  if a particular model is faster (top speed, in a velodrome, with a reasonably fit rider) than the standard boris bike available in London?
Neither Boris bikes nor folding bikes are exactly built for speed, so we can consider mainly the gearing. For this,  gear inches are a convenient measure taking into account not just the gear ratio but the wheel size. 

On Brompton folders (as a well-documented example) you have a choice of gearing but the default 6-speed gives up to 100 gear inches. That's quite high for a bike with small wheels and flat bars. For short urban journeys you might not make it into top on the Brompton. 

The Boris bike is a 3-speed with up to 60 gear inches. this is actually lower than the single speed Brompton. But it's not a stupidly low cruising gear. If you know the rear wheel size and tooth counts for the chainring(s) and sprocket(s) (plus any hub gears) you can use a gear inch calculator where you can also work in metric if you'd rather.

For many typical urban journeys, the time taken is determined more by the traffic than the bike. A bigger difference is trading off the unfolding/folding time against finding a Boris bike and then finding somewhere to park it, plus walking between where you park it and your real destination. You know the journeys you're likely to make better than we do, and so you're on your own for that. 

Another benefit of having a folder is that it's yours. You know you've got it if there's a problem like a tube strike when the Boris bikes will all be taken. You can also customise lighting and luggage to some extent.