Increase the number of threads available for Microsoft 365 Apps

 Microsoft 365 Apps that are deployed to multi-session environments are limited to two threads for multithreaded recalculation (MTR) and worker thread pools. 

1. Open Registry Editor.

2. Locate the following registry subkey:

   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Common

3. If the IdealConcurrencyValueOverride entry doesn't exist, right-click the subkey, select New > DWORD (32-bit) Value, and then enter IdealConcurrencyValueOverride as the name of the entry.

   Note: By default, the IdealConcurrencyValueOverride entry doesn't exist, and Microsoft 365 Apps is limited to using two threads.

4. Right-click IdealConcurrencyValueOverride, and select Modify.

5. In the Value data field, enter a number between 2 and 512 to specify the number of threads, and then select OK.

Reference: Heavy Excel workloads run slowly in multi-session environments - Microsoft 365 Apps | Microsoft Learn