Excel 2016 slow to startup

One of our users has a problem with Excel 2016 (Office 365 user).

The user has a map with about 15 excel sheets, which he wants to open one at a time. The first time the user opens a sheet it opens fast, but after about 2 or 3 sheets, it can take up to 10 seconds before Excel opens the sheet.

Now I already searched and tried several solutions, but until now I haven't found a working solution for it.

These things I already have tried:

- Uninstall Office 365 and reinstall it
- Restore Office 365
- Installed all updates
- Disabled all add-ins
- Disabled hardware acceleration
- Uninstalled the audio software (from HP)
- Enabled SuperFetch service (this was already enabled)
- Hi Cortana service isn't available on the PC, because it isn't available in Dutch yet
- Changed the standard printer
- Disabled settings for protected view en file blocking
- Ignore other application which uses Dynamic Data Exchange (DDE)
- Removed Excel roaming filed (%appdata%\Microsoft\Excel)
- Monitored it with Procmon, I don't see anything with that that could be interfering with Excel

It doesn't matter if the Excel sheets are stored locally, on a NAS or on OneDrive.
There isn't any other security software running, besides Microsoft Defender. 

The PC is only 2 months old, it's a Core i5 HP Pro PC with 8GB memory, 256GB SSD, 1TB HDD.

Is there any other solution I can try to solve the long startup time for Excel?