Obviously, what kills the memory of computers is mostly improvements and units. Here's a quick table of loading times during the elaboration of a scenario : Empty Map : 10 sec.
+ Cities : 2 minutes.
+ Railroad, roads, irrigation, mines : 5 minutes.
+ Improvements : 45 minutes.
+ 45% of units : 1h35.I simply can't put less units without disrespecting the balance of forces in that scenario. However, we can also get rid of many city improvements if we had more food, shield and trade to each tiles...

Do you know tricks to reduce loading time ?

Are C3C scenarios indicative of large scenarios? It doesn't take me anywhere near that kind of time to load C3C scenarios. :confused:

There's not a whole lot that can be done if you are not willing to work with a smaller map and fewer units.

to reduce the number of improvements you need try this: create uber improvements (Washington's Improvements, London's Improvements, Moscow's Improvements, etc.) that do the job of multiple improvemets. Set these improvements to require non-era techs that are unavailable and you don't have to worry about people building them in game. You may have to have multiple uber improvements depending on what you want in the city, but this method should allow you to have a minimal number of improvements in each city at the start.

That should work to help cut down some of the time, but the biggest issue is the units and probably the size of te map.

Yeah I agree Kal-El. The map is very large : 200x256. :(
I thought it wouldn't matter that much since most of the map is made of water (75%), however, that doesn't matter that much obviously. I'm pretty sure that the same level of improvements, units and cities in a smaller map wouldn't take so much memory. Obviously, the computer calculates things that takes more time with a large map.