In a nutshell: Our test Mac produced noticeably weaker performance across GFXBench's graphical tests when running Yosemite. The difference was sometimes as large as 66 percent.
Mavericks vs Yosemite speed testing: Internal IO performance
We checked internal storage transfer speeds with the two operating systems, using Intech QuickBench to measure read/write speeds with different data sizes.
Tiny differences were seen, which essentially evaporated after enough iterations and averaging. So whether with Mavericks or Yosemite installed, we saw the same over-achieving results from the little PCIe-attached flash drive, nudging 790 MB/s for sequential reads and 740 MB/s sequential writes. For small 4 kB files, random reads were around 17 MB/s and random writes around 64 MB/s.
In a nutshell: No discernible/significant difference.
In summary: Does OS X Yosemite slow down a Mac?
For processor- and memory-based benchmark tests, Yosemite was typically around 1 or 2 percent slower than Mavericks on our test MacBook.
For graphics-related activities and tests the situation was more complicated. In the two Mac games benchmarks, one game was around 20 percent faster in Yosemite while the other was essentially the same. But do remember that while double-digit increases sound impressive that may only be a few frames per second.
In other graphics tests such as the synthetic GFXBench suite Yosemite returned much less consistent results but tended to return lower figures, sometimes dramatically so.
And for data input/output as measured via the internal drive, results were realistically the same for the two operating systems.
Outside the lab
For matters surrounding the change in interface, this is harder to measure. More translucency with frosting effect in window frames and panels would suggest more graphical work required to drive the interface, for example, which might fractionally slow down the 'feel' of some older Macs.
One of the harder aspects to quantify is latency between your action and the resulting feedback from the screen.
This may take the form of input lag when buttons are clicked or windows picked up and moved. Subjectively on our MacBook, there were no perceivable differences here.
There may be other overall slowdowns due to the reworkings in the human-user interface, however. This user, for instance, has found that the reduction in window drop shadow makes it harder to easily discern the layering of windows, to quickly recognise the edges of open stacked windows. This is an issue common to the even drier and more two-dimensional Windows 8 interface, if more marked and annoying in the latter.
We also found the blurred font issue to be a productivity slowdown, since we sometimes need more time to peer at small, less legible writing, followed by more regular screen breaks to reduce eye fatigue. The blurry writing issue is less apparent on Retina-display Macs, but conversely makes our 2012 15-inch MacBook Pro's screen (1680 x 1050) too tiring to view with OS X 10.10 installed.
Sign up for CIO Asia eNewsletters.