It does not matter how fast it is growing. That it is growing and old levels become obsolete is the main result of diminishing returns.
The thing is - you can't get 22 instead of 2.2 in 15 minutes due to dates having a fixed time, so you can exactly count your multiplier increase speed and with higher total multiplier it won't increase for long, it will get to a constant one that you can't reduce anymore. And it does not matter how long it will take to get 1 million time bonus because you still have to use optimal resets to get there faster, if you ever want to get there or at least if you want to have a higher bonus than the others who are in the same position.Diminishing return is not something implemented in a game, but rather a mathematical problem.
You can farm a bonus of 2,2 in 15 minutes. This will take you about 13 years (!) to get to 1 million time bonus. And with that you will still be far, far away from twins. (And no, getting 22 instead of 2,2 in 15 minutes is no difference. 1,3 years or 13 years are both too big for real play time).
Again. Diminishing returns term can't be used alone. Diminishing returns has two parameters - one that you are trying to increase and the other that you are spending to increase the first one:That you will gain less and less effect from the same resource cost (2,2 constant every 15 minutes very active play) is the diminishing return. Not the reduction of the resource gain itself (2,2 reset bonus).
Diminishing returns for base multiplier on invested time does not exists.
Diminishing returns for relative multiplier on invested time does exists.
When you are just using a term Diminishing return you are combining those two and it sounds incorrectly. First case (diminishing return of base multiplier on time invested) defines which place you need to farm to progress faster. If it existed then you would have to move to higher levels to increase your gains because lower levels would become obsolete the higher your base multiplier is. Right now because it does not exists 2.2 multiplier per 15 minutes is the most optimal place to farm and it this optimal place does not change. Second case (diminishing return on relative multiplier on time invested) defines the resulting effect of your farming, it slowly diminishes with higher multiplier and you can do nothing about it.
Here you can also see two types of diminishing returns:Time warp has no diminishing return, but a capping and slightly increasing costs.
Diminishing returns of total multiplier on invested time warps - does not exist because each 8x you buy increases total multiplier by the same 8x.
Diminishing returns of total multiplier on gems invested - exists because cost of each next time warp increases when you have more of them.