Absolutely hearsay here but a family member worked for SNCF as a mechanic for TGV trains (but not the locomotive) and has said that the pantograph was the limiting factor they had designed the last generation around. That it was designed as a wear part and the top and optimal speeds were chosen based on thresholds for unacceptable wear on the transmission lines themselves, which are much more difficult and expensive to replace. He told me never to expect TGV speeds to increase significantly until we saw them ripping out the powerlines and replacing them with an entirely new system.

Again I don't know if this is true! But it seems reasonable enough to me. That conversation was about twenty years ago and plenty has changed with TGV in the mean time but sure enough no top speeds have increased on any line in the system in that timeframe, and new lines are deployed with the same speeds as the older ones.