Also bear in mind that for active applications, data volumes increase over time, and that could also degrade performance. The larger your data volume, the more important it is to have the correct attributes indexed in your domain model.
See also the forum post here: https://forum.mendix.com/questions/9474/Performance-Testing-for-Mendix-Applications I know that performance has the attention of Mendix. If you have specific examples it would be nice to discuss here in the forum.
Regards,
Ronald