System Tweaks
Windows
For a modern Windows 10/11 setup, required tweaks are minimal. Ensure that you are using 'High Performance' or ideally 'Ultimate Performance'. which, if not available, can be activated by opening a command prompt and pasting:
powercfg -duplicatescheme e9a42b02-d5df-448d-aa00-03f14749eb61
You can then select it from the usual power plan dialog. For best device performance, remember to use the ASIO drivers that were included with your audio interface. Consider disabling your network card during mission-critical recording. For further tips see Microsoft's Unofficial Windows 10 and 11 Audio Workstation build and tweak guide. Of particular interest is Part 3 which discusses what not to do (including not setting your processor scheduling to 'background services').
MacOS
Performance should be excellent out of the box. Remember to disable energy saving (System Settings > Energy on recent macOS, or System Preferences > Energy Saver on older versions) so that sessions are not interrupted. Consider disabling your network card during mission-critical recording. If you are experiencing any audio issues while in your DAW, for tips see Focusrite's optimization page.
GNU/Linux
Linux users should visit my Arch Pro Audio Guide. Performance out of the box on Linux is getting better with every kernel release and is on par or better than Windows. As a starting point, ensure that your username is set up for realtime privileges (step 3), that the threadirqs kernel parameter is set (step 4) and that you are running the performance CPU governor (step 5). Check that your kernel is fully preemptible too (see the guide's introduction). To make things even easier, consider using a dedicated audio/video distro like AVLinux or Ubuntu Studio where everything is already set up for you.
Cross-platform
The good news is that REAPER and ReaClassical run on many different architectures and operating systems. You should always be able to open up a project on another machine even if it is running a different OS. Plus, ReaClassical includes various mastering-grade plugins in JSFX format that run on all architectures.