Currently, the day length varies from 20 to 35 RL minutes per game day, depending on the game year.
I think the game speed should work differently:
When the game world starts (no matter what year), the game day should be 35 minutes. It should accelerate to 20 minutes per day. Maybe 30 minutes after 5 years, 25 after 10 years, 20 after 15 years (until the end of the game world)
When the game world starts, there is a lot to do for the player. Later on, say last 10 years between 2020-2030, players just upgrade / swap some aircraft, occasionally add a flight. There is not really a lot to do. Faster speed later on would be an antidote to boredom.