Not at all. My audio / MIDI interface is a Tascam US-122, so same hardware on each platform. It has Windows ASIO and OS X  drivers and was totally unusable on the Tecra and works superbly on either Mac. I've seen this countless times before over many, many years; for some reason PC's are sluggish when it comes to real time MIDI processing – hit the keyboard and at some point in the future the soft synth or other device will trigger. There is good reason about 90% of pro studios use Macs! I have yet to see Cubase work perfectly on a PC – it is renown for being flakey as hell - even if you can get the latency to be anywhere near acceptable it is prone to invoking the BSOD. Pro Tools fares somewhat better and Logic (the best of the bunch IMHO) is now Mac only (Apple bought the company). Macs are way out in front in this particular arena and always have been since the day they replaced the old Atari ST in studios. I've not tried Cubase later than SX so have no idea if it is any better these days - from what you say it may well be.

My theory was always that it wasn't the hardware but the way Windows multitasks - what is right for a business networking environment was simply not right for real time music processing. This gave even the old Atari a huge advantage over pretty much any Windows prior to XP - it had a built in MIDI interface and knew well how to talk to it punctually - I used a ST running Cubase in a fair sized project studio for many years (lots of MIDI kit and an ADAT). Macs running MacOS (i.e. prior to X) were pretty useless at multitasking so coped well with a sequencer as the foreground app got pretty much all the CPU -  these old Macs would be perfectly happy running a sequencer and did so without latency, but would often glitch or crash if you tried to do anything in the background at the same time. I'm sure the two platforms are moving closer together now as the raw power is there on each now which can probably compensate for OS bloat and poor design with regard to real time processing.

Tony.