Possible breaktrough!
I have found a way to limit trade in the game, and it reduced the seconds needed for a day to advance to 12 seconds, nearly as little as the seconds needed for the day to advance with trade disabled (9 seconds).
This is what I did, I changed defines.lua which you can find in the common folder:
defines = {
...
IC_TO_CONSUMER_GOODS = 5.0,
IC_TO_SUPPLIES = 7.0,
CONVOY_BUILD_COST = 2,
CONVOY_BUILD_TIME = 40,
ESCORT_BUILD_COST = 3,
ESCORT_BUILD_TIME = 50,
LEADERSHIP_TO_SPIES = 0.05,
BUILDING_REPAIR_SPEED = 0.1,
LEADERSHIP_TO_OFFICERS = 1,
THREAT_FROM_CONVOYS_MODIFIER = 0.1,
CONVOY_CONSTRUCTION_SIZE = 10,
MAX_DAILY_TRADE = 5 - Originally, it was 100
...
Besides improvements in speed, al those countries will spam you less with trade proposals!