For anyone who's interested, below is a list of boot times for an iMac7,1 running 7 different versions of OS X.
10.4.11 = 26s
10.5.8 = 31s
10.6.8 = 32s
10.7.5 = 41s
10.8.4 = 39s
10.9.5 = 43s
10.10 = 49s
The timings are a measurement of how long it took to get from a cold boot to a usable desktop.
I used a script to do the timings (
https://sourceforge.net/projects/osxcoldstart2desktoptimer/) which is better than using a stopwatch - but there's still probably a margin of error that I'm uncertain of.
As you can see, there is a gradual increase in boot times.
I'm not complaining particularly, more features seem to always equate to longer boot times. I only did the timings because our users are constantly telling us that "...the Macs are so much slower than last year". I just wanted to confirm what was happening.
The iMac 7,1 is our oldest Mac. I have a vague memory of it being a reasonably quick machine when new (late 2007) although it seems quite slow now. The test timings appear to confirm this. It is however, a good test machine since it can run every OS from 10.4 to 10.10.
The Mac involved was installed with a base OS, and no other applications. It had been initially installed with 10.10 - so the hard disk partition scheme reflected this (it contained a 10.10 recovery partition).
I've not updated our users to Yosemite yet... but probably will very soon.