When technology switched from HDDs to SSDs, the performance bump was pretty clear. Now Apple must add too much graphics gimmicks to make any SSD Mac obsolete.
Heck, we can even disable translucency. This allows Yosemite feel just like Mavericks in terms of speed. Also, Apple cannot compromise performance in OSX because they still sell brand new 4GB RAM Macs. They still sell HDD Macs (iMac and Mac Mini).
This said, I wouldn't feel so worried about OSX performance in the long term even with base model Macs. As long as you can upgrade to a SSD, you'll be fine for years with just 4GB of RAM. There are people still using their 2GB Macbook Airs. This is suitable thanks to the SATA-III SSD.