What makes a mental math game fun enough to keep playing?

I’m a math teacher and I’ve been developing a free mental math game called CalcuLoop.

The idea is pretty simple: instead of doing mental arithmetic as a traditional exercise, I wanted to turn it into a speed/arcade challenge.

You solve calculations as quickly as possible, try to beat your previous scores, and gradually increase your speed and accuracy.

There are several game modes, including:

• progressive levels for training
• a speed test to measure your calculation speed
• timed combo-based gameplay
• increasingly fast calculation chains
• a 100-calculation challenge
• online leaderboards

I’m particularly interested in hearing from people who actually enjoy mental math:

What makes you want to keep playing a mental math game?

Is it:

• beating your own high score?
• competing with other players?
• increasingly difficult calculations?
• very short sessions?
• leaderboards?
• unlocking things?
• simply trying to become faster?

And conversely, what makes you stop playing?

I built CalcuLoop myself, so I'm still trying to figure out what makes this kind of game genuinely fun rather than just another math practice tool.

If anyone wants to try it, it's free on Android:

https://play.google.com/store/apps/details?id=com.benmaths.calculoop

If you're on iPhone, you can also play it for free directly from the web version:

https://benmaths.pythonanywhere.com/

I'd genuinely love to hear what you think, especially from people who enjoy doing mental calculations for fun.