Hi,
After trawling all of these "fixes" for laggy Safari (and graphics in general on Yosemite) for my 2014 MBPR, I accidentally discovered the real fix, for me at least.
I have FileVault 2 on, usually, always have. I decided to disable it (and have it decrypt the SSD), just for sh!ts 'n' giggles.
Safari is now instantaneous, just like Chrome, and graphics around the system are generally snappier than with FV2 enabled.
I now have created a disk image (dmg) which is encrypted separately, so anything personal can be kept in that, and I'm not too fussed the whole system isn't encrypted now, as it's far more responsive, even on a system as fast as this.
So try disabling FV2, you can always enable it again afterwards if it doesn't fix it for you, but the difference for me is remarkable, and is definitely thee fix for my laggy Safari.