Here are the official words and guidelines from the Microsoft Project Team:
Project admins can configure Project Online to roll up timephased reporting data to different levels of granularity. They can choose to roll up their data daily, weekly, monthly, or by fiscal period and retrieve the data through the timephased OData endpoints:
A key benefit this provides is that it can help to improve report generation performance, especially in organization that have a lot of Project Online reporting data. While retrieving task and assignment timephased reporting data by day was the only option available previously, many organizations do not need this level of granularity. Being able to retrieve their data on a broader scope (such as weekly or month) can greatly improve report generation performance by reducing the amount of records that need to be downloaded. For example, instead of needing to retrieve 300,000 records when set to daily, setting to monthly could reduce this to a much lower number, such as 10,000 records.
Additionally, other benefits can include simplified report building, and reduced storage requirements.
How do I configure this setting?
- In Project Online home page, select Server Settings.
- On the Server Settings page, in the Enterprise Data section, select Reporting.
- On the Reporting page, in the Timephased Data section, select the option that you need:
- By fiscal period
Tip: If you don’t use any of the timephased Odata feeds, we highly recommend setting the Timephased Data option to “Never” which provides the added benefit of much faster Reporting Publish queue jobs.
Planning considerations I need to make
- Republish existing projects – After changing the Timephased data setting, it will only take affect in an existing project if it is republished.
Note: If you have many projects to republish, you can use the Office 365 Project Online CSOM Tool(ProjToolV2) to programmatically publish your Project Online projects. You should plan to do this at a time that will be least likely to impact your organization.
- New instances – 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.