Hi, kjellkri
Welcome to the Microsoft Community to share your questions!
Based on your description, your network speed is fast, but there may be delays in OneDrive's file synchronization or bottlenecks in data transfer. Excel needs to download the file content from OneDrive, and if the file is large or the synchronization status in OneDrive is not complete, it may cause delays when opening the file.
Additionally, the data synchronization method between OneDrive and Excel on iPad may differ from that on Windows. The Windows version of Office utilizes local storage more efficiently, avoiding the need to download files from the cloud every time, whereas on the iPad, it may need to read from the cloud each time, which increases loading time.
OneDrive stuck on Processing changes - Microsoft Support
Furthermore, if there are too many cached files in OneDrive, it could cause the app to start slowly or files to load slowly. Therefore, you may try resetting OneDrive and see if your issue is resolved.
Reinstall OneDrive - Microsoft Support
I hope the information I've provided has been helpful and thank you for your letter
Allen L- MSFT | Microsoft Community Support Specialist