I myself didn't actually try these commands but iirc, I think campaign.multiply_campaign_speed is the one that speeds up the game from what I saw Strat playing the game, but the command also makes the daily ticks like wages and influence, ect. faster as well, so not exactly what I suggested. Does the command also make your and AI parties twice as fast if you speed up the game x2 ? If yes, my idea would be much harder to implement and lowering the amount of days in a year becomes much more feasible (probably, idk if its something easy to change).
You're right though I didn't really think about the impact on performance. Although making it a slider with a red text underneath when you choose faster game speed warning people that this setting taxes your performance can be a band-aid solution I guess.