In my opinion it is good idea to introduce such maps (Maps time speed x6, x8, x10)
for example for me x4 is slow, especially in the begining when Infantry has level 1 - speed 1.00.

Moreover economy can be improved in the cities - for example new building
1) power plant/nuclear plant (+5% resources production)

4x time I won 3 times alone without gold several times in coalitions. x4 is slow that is all.
Of course if you have 150 cities and 150 units it can be quite fast. For example I had battles in North America, South America, Europe and Africa simultaneously
and also fought vs multi accounts player

x8 time should be easy. If there is server problem. It can be run on smaller maps (20 players)