Which VSTi uses more CPU than before for you then? This is not a general problem. Some VST might perform better with the static buffer option enabled (see https://forum.renoise.com)). This option was enabled by default in Renoise 1.9.
See https://forum.renoise.com/t/build-new-system-dual-or-quad-core/23899 for more details about the the quad core thing please.