Performance of C7 may be worse on old systems, but note that testing of Cubase goes on with HP Z Series workstations, which are Xeon/SSD professional workstations with custom motherboards and what not. Maybe it is time to upgrade machines.
As steinberg claims on C7 page: http://www.steinberg.net/en/products/cubase/whats_new.html
“Made for each other: Cubase and HP
Finest software requires finest hardware. That’s why our best software specialists rely on the world’s leading PC hardware manufacturer. Cubase 7 is quality-tested with HP’s professional Z series workstation computers, ensuring highest performance and maximum reliability packed in a rock-solid chassis. Carefully selected components optimized for recording, editing and mixing allow for efficient audio data transfer rates throughout the whole system. Be it the industry-proven Intel XEON processors or the ultra-fast SSD drives, HP Z machines squeeze the best out of Cubase, speeding up your entire studio software environment.”
From this it is clear that C7 is made for HP workstations. So if there is problem with performance, then one is advised to get one HP Series 7 workstation. I bet that those who have one of those, have no problems with performance at all.
So there is solution for those requiring absolute performance. And clearly all others may have to deal with some performance issues. Software gets more CPU hungry with every new version, thats development.