Hello Bartosz,
I think i would start with what is the expected performance, so what are the KPI's that you should met, try to agree upon a definition of the kpi and how to measure it first, then implement your measure and start from there to adjust things.
After this there are a lot of things to look at within mendix think about the following:
- Retrieves
- Commits
- Loops
- Xpath Expressions
- TaskQueues
etc.
Take a look at the mendix documentation and learning paths to learn more:
documentation:
https://docs.mendix.com/refguide/community-best-practices-for-app-performance/
Learning paths:
https://academy.mendix.com/link/paths/39/Optimize-the-Performance-of-your-Apps
https://academy.mendix.com/link/modules/371/lectures/3011/4.2-Performance-Impact
Hope this helps, but also be honest when it will not work with mendix it is always better to advice good than deliver poor!
Good luck