raw disk performance

As so many systems have some degree of fragmentation, ask yourself, what disk I/O happens most frequently? - - random reads! After a good defrag, sequential reads should dominate.

Boot time is nice if it is really low, but as that occurs only ONCE per session, so don't get myopic on that metric - -
focus on what occurs most frequently - - imo, launching any program[*].

Some background on HD performance.
There are three measurements for a single I/O:

move the arm to the proper location, (ie SEEK)
let the platter rotate to the right sector (ie rotational delay)
DRM the data into (out of) memory, (ie read/write)

Years gone by when we had multiple platters on one HD, there was a HEAD SWITCH that followed the SEEK.

If the SEEK costs 100 units, then rotational delay will be typically 10 units and the final read/write will cost 1 unit.
Seek times are relatively fixed, and the read/write time is trivial compared to the other two.
It ends-up, that rotation delay is inversely related to RPM; the faster the drive, the lower the delay.
Laptops usually opt for 5400 rpm as it saves battery life.

There's a great tool Defragler, which allows you to pick precisely which files get compressed into contiguous sectors.
Run that under an Admin-ID and defrag EVERY DLL or *.EXE that is more than one fragment.
This will vastly improve [*] noted above.
Now that the programs have been optimized, run a full defrag to fix the other stuff.

Oh yea, don't minimize the degradation that a fragmented pagefile will cause you!
Look for PageDefrag and run that once in a while

But that doesn't address caching! Right. If the raw performance is poor, then adding a cache is nice
but the cache updates are still (relatively) poor.