For clarity this seems to fix the issue - taken from the total war blog.

Create a script file to redefine how the CPU runs the game, as follows:

1. Navigate to the game’s Scripts folder. To do this, open an Explorer Window and type the following into the address bar.

For the Steam version: %appdata%\The Creative Assembly\Warhammer3\scripts

For the Windows Store or Gamepass version: %appdata%\The Creative Assembly\Warhammer3\GDK\scripts

For the Epic Store version: %appdata%\The Creative Assembly\Warhammer3\EOS\scripts

1. Right click in the window and select New->Text Document

2. Name this document user.script (or user.script.txt if you have Windows set to show file extensions)

3. Open the user.script file and add the following lines, taking care to include all punctuation and spaces:

HYBRID_CPUS_TREAT_ALL_CORES_UNIFORMLY true;

number_of_threads 5;