My subjective take on Windows 10 (And a really old computer that was never built specifically to run a DAW)…
CuBase 7.5 seems to be the gold standard (of CuBase versions) in terms of raw system resource efficiency. I personally don’t see much difference from this to the latest Version 9 in terms of system resource efficiency, but in really large projects the tiny differences might add up on some systems.
Advantages to upgrading to 8.5 or beyond would be gaining ‘instant rendering’. That makes it easier to render out MIDI or Instrument tracks into pure audio, which can recover some head-room to begin new VSTi tracks. I.E. Just pick a MIDI or Instrument track and instant render it to audio (with or without the insert effects in the mix). The MIDI/VST version just rendered can optionally get auto disabled and hidden during the process if desired. The process usually only takes a few seconds, as opposed to the old way of having to ‘bounce’ a recording in real time, or ‘exporting’ offline mixes and then manually pulling the results into the pool, and then onto an audio track.
On my older computer, instant render has been wonderful! It has actually allowed me to keep using this old system and work with much larger projects than I ever could before.
8.5 also added VCA faders, which are of no value for virtual instrument MIDI tracks (One can still link MIDI faders together though…just not via VCA), but can really come in handy for audio. So if you’ve rendered a lot of stuff in place, it’s nice to be able to link them together at relative volumes on a ‘single fader’ without using a more processor intensive (at least for a particular cpu core) group bus.
9 adds some very useful things to the main mixer. Up until this version there was no memory of steps one had taken while mixing, thus no way to ‘undo’ things. Version 9 makes it possible to ‘undo’ and go back to previous steps you might have done in the mixer. Along with this you get some revamped/updated plugins, and the ability to run an editor in a split screen (lower zone) of the main project window. You’d also gain a dedicated ‘sampler track’, which can really come in handy for quickly making and/or adding various sound effects on cue.
For me, there has been very little noticeable change in the overall performance from 7.5 through 9 with one pretty big exception. The latest update/patch for CuBase 9 changes to a new video engine (no more Quick Time for ‘security reasons’). I personally don’t use Video inside CuBase all that often so it’s not a big deal here, plus what we do get is still usable for me (just not as efficient), but if you do rely on Video you might want to try out a demo first, or wait until 9.5 and see if they optimize the video engine a bit more.
Actually, it’s always a good idea to try the Demo first and see if it’s of value to you Personally, I’ll usually give demos a trial run on a different system drive that I keep laying around for trials and betas (just a precaution to prevent the possibility of messing up my regular set-up).