Depends on a lot of metrix I would advise to maybe try out the APM tool from Mendix? Best way to get some qwick wins is trying to reduse commits and remove old and not used microwflows and pages.
We faced the same issues here. With a performance decrease of 100%. Microflows that run before 1 hour, do run now for 2 hours.
We did not add any functional change for the migration.
It is know by Mendix support and issues are related to:
1) List operations
2) A new function that is added in Mendix 5, that creates overhead
3) Conversion float (deprecated) to decimal
Hopefully Mendix will solve this, so we can finally upgrade from Mx 4 to a higher version.