It has very little to do with warm muscles - muscles should be warm whatever the weather if you're working hard enough and everything to do with air density.
The greatest slowing force on you when time trialling is drag. That's why riders will ride in inefficient positions in order to reduce drag - the pay back of reduced drag is worth the loss of power. The denser the air the greater the drag. At a fixed pressure the higher the temperature the less dense the air (Boyle's law). In summer this is partially offset by the high pressure systems that tend to be associated with settled weather - high pressure=denser air, but the warmer air is still less dense and undoubtedly improves speeds significantly.
The ideal weather for a really quick time is a low pressure system moving through in a period of warm weather when you get low pressure and warm air together thus very low density air. Problem is that low pressures tend to be associated with wind which is not good for quick times!
Traffic makes a huge difference even though you are not allowed to draft vehicles. Traffic passing will give a drag and as Andrew points out the bigger the vehicle the bigger the drag. At least one of the really 'fast' courses is rubbish for early morning events as the only reason it is fast is that it has nose to tail commuter traffic (with supermarket trucks mixed in) travelling at 50mph for the first half of the evening events. If you set off in that I reckon it's worth 30 to 45 seconds over a 10 mile course for the fast riders even compared with the later riders in the same event. Some events you need to check the ferry timetable to be sure when the 'quickest' conditions are or have a mate at Catterick who can tell you when the army are trekking up or down the A1.
George if you can't knock out a 19-minute ten in your first couple of rides I'd give up