Mmm, let's take Peterborough to Stevenage as a example shall we?
East Coast run at 125mph for most of the journey and it takes 30 minutes.
FCC run between 75mph and 100mph and the journey takes 50 minutes calling all stations.
If the slow lines from Welwyn Garden City to Peterborough were 100mph only then this would certainly mean some time saving of maybe 10 minutes so FCC could do the trip in 40 minutes rather then 50 mins.
I don't wish the slow lines between Welwyn Garden City and London to be upgraded as they are used quite a lot by FCC metro services and it would mean the faster services chasing the locals!
Stevenage to Kings Cross takes:
20 Minutes by East Coast
30 Minutes by FCC (stopping FPK, KGX)
40 Minutes by FCC (stopping KBW, WLW, WGC, HAT, PBR, FPK, KGX)
60 Minutes by FCC (via Hertford)
Those times are approximate and fluctuate a little bit during the peaks.
Some of the current 12 car evening services have Hitchin as their first stop (fast line to Hitchin) then I think they use the slow line to St Neots as the next stop. I have seen these leave Hitchin on the down slow. Presumably some of these will also call at Biggleswade after December when the platfrom extentions are completed.
Some of the twelve car services will remain at the same times, (from origin station), whilst some will be changed completely. FCC are considering two options for AM 12-car services, both of which have 6 PBO-KGX services. These are more or less the three that run now plus three new trains stopping all stops to SVG then FPK and KGX.
In the PM peak, FCC seem to have made a decision as I have only seen one option. SVG is set to gain two twelve car services, both of which already call (1809 and 1839 at SVG), the latter of which is chronically over loaded. As far as I can remember the other 12-car trains are those already running.
Take that with a pinch (or a bucket) of salt though as there's still months to go and it's entirely possible that it could all change.