Generally speaking the performance issues / instability problems tend to be related to basically a few things..
1) The general issues with Yosemite, memory leaks and 'runaway' routines from time to time... Further updates to Yosemite/LPX should fix most of these over time.
2) LPX not being fully optimized (Yet!) for Yosemite... The graphical/GUI side seems to need to most tweaking....
3) Changes made to how LPX handles graphics/GUI compared to LP9. See #2
4) 3rd Party Driver issues.... requiring updated drivers/firmware for 3rd party External Hardware..
5) USB3 /USB2 related issues... again requiring updated drivers/firmware for 3rd party External Hardware..
The following tips seem to 'calm' Yosemite down somewhat..... providing better / reliable performance overall (including with LPX)
Guide: How to solve Yosemite memory leaks and CPU usage
Reasons for these issues?
Personally I think the main reason is... Apple is simply bringing out too many "new' OS X features without fully fixing the existing issues in prior versions... In other words the recent 'Once a year' update/upgrade cycle is just too fast.. and we, the end users, end up being unpaid beta testers for most of the time... and many known bugs just don't get fixed at all... or after extended periods of time... by which time other bugs rear their heads and so the cycle goes on..
Note: In the past OS X went through major updates every 2-3 years as a minimum..and so the updates had time to 'settle in' and get fixed before the next major version was released... and I believe OS X had far more time 'behind the scenes', being tested prior to public release... which simply isn't happening these days.
There will be other reasons of course.... One of which is probably related to the fact that cheaper/less well supported/less well tested and designed hardware is being released onto the market because the user base of Apple devices has risen making it worthwhile for some Manufacturers to release such cheaper products... as they will sell in greater numbers than before.
As I have said before.. it always strikes me as odd when someone spends $2K+ on a Mac and then spends only $40-$90 on a cheap midi/audio device...and they then wonder why it's performance is terrible/unstable. Its akin to putting $50 tires on a $200k Ferrari and then being apparently surprised that it only performs like a Kia....