OK so on my Windows 2022, ~i had to take possession of, and change the permissions of this key in RegEdit:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Search\Gathering Manager\And alter the value of the string:
DisableBackoff to equal 1 and then restart the Windows search service which transformed it to searching at around 3000 emails per second.
It is now down to 300k emails remaining to search after a few minutes which is a far happier situation.