slow bike...is it the chain? I have two Specialized Langster (one brand new 2013 model (54 cm) and the other a couple of years old unique London model (52 cm)) - both are single speed. Got sick of getting punctures on my daily commute so on Monday I got Durano Plus 700x25c tyres - the bike shop owner said go for 25c as the 23c is good but for daily commuting he would recommend 25c - he did acknowledge you will go slightly slower but not that you will really realise.

Well, he was wrong and I have realised! Anyway, the problem is the London model seems to be a lot slower then the the one - both are slower now but the London one lot more. 

I am able to really pedal the heck out of the London one but it just does not seem to go too far I know I can pedal more but there is no need as it just does not cope to go further (hope I am making sense as it is hard to explain!). Could it be the chain is just not good enough on the London model then the other bike? Or something else - there is nothing much else I can think of as it just has one chain and no gears and stuff - except for the new tyres??

One point - the chain is painted (as it is a unique London model) so could it be that? I only bought the London model two weeks ago and am only now riding it around now for my daily commute.
One note is that the gearing is different, assuming they're still stock.

The Specialized Langster London has a 42t chainring and 16t cog, the 2013 Langster has a 48t chanring and 17t cog.  With 700x25 tires that means they are 69.6 and 74.6 gear inches respectively.

What that means is that spinning at 100rpm, you're going ~22mph on the Langster and only ~20mph on the London.  So it will feel slower when you're trying to go flat out.