Time Trial / Time Trialling / TT

On the subject of 25s... I've got my first one since 2011 tomorrow and I'm wondering how to pace it.
How much of a drop in speed would you normally expect between a 10 and a 25, all else being roughly equal?

In my sample size of 1, taking my PBs I'm 1kph slower at the 10 vs. the 25.

I generally do one 10 per year though.