A new feature is "in deployment": Configure rollup of timephased reporting data in Project Online
It will allow you to set the granularity of the data delivered by the timephased OData endpoints, thus giving the ability to reduce the amount of data that is delivered by the endpoint.
All details can be found here:
https://support.office.com/en-us/article/Configure-rollup-of-timephased-reporting-data-in-Project-On...
Be aware of the note at the end of the article:
In new Project Online instances, note that the Timephased data default setting is Never. Make sure to select the appropriate Timephased data setting after creating your new instance to prevent the need to republish all your projects at a later time. Do also consider the tradeoffs associated with a more granular setting e.g. Daily vs Monthly with regards to the time it takes to Publish as well as pull down the timephased OData feed
