Hi Oli,
To clarify the situation, we’d like to confirm the following details:
- As the local files and older presentations work fine, does the high CPU load only occur to the presentations which are saved and synced online like OneDrive or in OneDrive for Business client? What about the files created in PowerPoint online?
2.How long does the High CPU usage last? Does it start from the second you open the PowerPoint till you close it or it just lasts for several minutes?
3.When did the issue start to happen? Although you have experienced an update recently, the new update seems not fix the issue.
4.A screenshot of the PowerPoint version as below. Check it via File>Account.

Regards,
Kimi