The computers I use now are far more capable than the ones I used in 2005, in terms of CPU, RAM, disk space, GPU, and anything else that should affect an app's boot time. Today, Photoshop takes just as long to boot as it did in 2005. Yet the features I use today are almost identical to those I used in 2005, the only exception being the ability to import camera raw images. (And I'm sure that the binaries for importing camera raw images don't come anywhere close to accounting for all the bloat in that time.)
So, my computers have gotten better and better, while the demands I place on them have remained basically constant, yet boot time stays the same. The only thing I can think to blame is bloat.
I imagine load time hasn't improved so much because that's not really what they are optimising for.
You could of course still run an old version of Photoshop on a modern PC if startup time is really that important to you.
I think it would be interesting to compare performance of Photoshop CS5 with Photoshop 6 (the first version I used) on a modern PC.
I would expect of course that PS6 would start up almost instantly whereas CS5 would take a few seconds.
However it would be interesting to also take a relatively large image file of the sort of size that a graphics designer might actually work with (let's say 500MB or so) and apply a bunch of the same filters across the whole image (in such a way as to produce an identical result) on the same computer and time them both.
My guess would be that CS5 would be significantly faster which may be partly due to optimisations made at load time. Not sure if anybody has actually measured something like this though?
Of course Photoshop has plenty of new features each version. Just because you don't use them doesn't mean other people don't. It's a known problem: Everyone only uses 10% of the features in software but everyone has a different 10%.
Disk speeds have increased greatly in the last 7 years. Compare an 80gb 7200rpm hard drive from 2005 to a 1tb 7200rpm drive from now and you'll see a very large sequential read/write speed improvement.
So, my computers have gotten better and better, while the demands I place on them have remained basically constant, yet boot time stays the same. The only thing I can think to blame is bloat.