One of the things you run into with audio production software is that in order to maintain accuracy with respect to phasing between channels, they require very accurate timing, this usually comes at the expense of very high interrupt rates and context switching and under the best of circumstances this is hardware intensive, more so with older processors that don’t have single instructions for storing the entire register set or restoring the entire register set to the stack with one instruction as many modern CPUs do. So hate to say it but you may need to upgrade to something less geriatric. You might also try a real-time kernel, it might allow the application to keep things sync’d up with less hardware interrupts though it will rely more on software interrupts do do the same. Unfortunately, I have found that while I can get the performance I require on my six year old processor using a realtime kernel, it has come with a sacrifice of stability, that is to say real time on my hardware at least has not been terribly stable.
I’ve had Windows and Linux installed on the SAME drive for decades and don’t have this issue. Install Windows first because it WILL fuck up the EFI boot partition, that’s inevitable because Windows sucks, then install Linux, use the manual partition option and simply select the existing EFI System partition for the EFI and DO NOT mark format, Linux will then install and leave the Windows boot loader in the EFI partition undisturbed.
Linux WILL overwrite the boot block to start grub instead of the Windows boot loader, but most Linux distros will automatically add a chain boot loader entry to the grub menu to allow you to boot Windows, at least Debian and Redhat derived distros will do this, probably a more manual process in Arch derived distros.
If it does become necessary to install Whendoze after Linux, you can use boot-repair to automatically fix the EFI System partition Windows fucked up or you can boot off of a flash drive, and fix it manually.