Apparently, it takes 100% CPU utilization for the Mac App Store just to check if there are updates. So far it won't show anything for me and I eventually have to force quit the App Store. I was just going to update iTunes as I won't update to 10.10.4 until I've done a more recent complete back-up. This is the point Apple's software quality has come to. Their huge thing was having a new music service that came out yesterday. During that day I was at no point able to update iTunes to use the new music service. Can you imagine how Tidal would have been treated in the press if that were the case with their service? And how can just checking for an update take up so much processor usage? My computer sounds like it's ready for take-off with the fans blasting each time I try opening the App Store. This is why I do complete back-ups before ".1" updates now, which is another sign of Apple's software quality decline. Maybe they need to bring back optical drives and send out discs because they have never gotten the hang of Internet services. In fact they are going backward. Updates worked much better under the Software Update app versus the App Store. Spotify updates itself automatically all the time, and it's working on top of Apple's technologies. Apple controls every part of my eco-system: the Mac, the OS, my wireless router, but it just doesn't work anymore.
Sorry for your troubles. I find the MAS to be very reliable, and updates go quickly. I don't know the CPU % when it's running, but it has never had any effect on the performance of my system. I hated Yosemite when it first came out but mostly due to cosmetics. I've had a few non-critical bugs that rubbed me the wrong way, and Yosemite has been my least favorite version through it's cycle until today. As of 10.10.4 there only 2 small bugs that have bothered me. In fact these 2 annoyances may have been quashed but I will need some testing time before I know for sure. If there is another point update, and it that update shows further improvement for me, I may not even upgrade to Cappy for my main boot drive until it's near the end of it's update cycle. I currently boot 10.11 on a test drive only.
Somewhere along the way I developed a laggy mouse cursor (BT Mouse only), and I'm annoyed by the USB drive not ejected properly notification upon waking from sleep. These 2 things alone are my only issues and they are minor annoyances that I can live with.
I don't question the difficulties others have had, but I'm not personally seeing this massive decline in the quality of OS X. I hope you get your issues worked out soon.