People who have done a clean install (you must delete all partitions and backups) do not have unique problems, so if anyone has big problems with Yosemite, it is their fault for being comfy and choosing upgrade, especially from OS X versions from years ago. The upgrade process has many flaws even if done correctly. These statements apply both to OS X and iOS. Apple has never cared for upgrade scenarios, they care about the bleeding edge and so should you.
If you absolutely cannot clean install,
-repair permissions from Disk Utility
-thrash and empty trash for everything in Library/LaunchAgents, Library/LaunchDaemons and Library/StartupItems as these hog CPU, RAM and HDD/SSD at startup and during normal running
-disable all Extensions and plugins from Safari (use Safari, Chrome is slower and worse at typography) Preferences, Privacy except the ones you absolutelly cannot browse without, e.g. Flash (for some, although 99% of video websites work without flash - did you know that? try it; also Flash hogs CPU, RAM, fans and website loading like crazy)
-enter Terminal and type sudo periodic daily, then sudo periodic weekly then sudo periodic monthly
After these reboot and remember to do them once a week or at the very least once a month. Enjoy the blazing speed.
If you absolutely cannot clean install,
-repair permissions from Disk Utility
-thrash and empty trash for everything in Library/LaunchAgents, Library/LaunchDaemons and Library/StartupItems as these hog CPU, RAM and HDD/SSD at startup and during normal running
-disable all Extensions and plugins from Safari (use Safari, Chrome is slower and worse at typography) Preferences, Privacy except the ones you absolutelly cannot browse without, e.g. Flash (for some, although 99% of video websites work without flash - did you know that? try it; also Flash hogs CPU, RAM, fans and website loading like crazy)
-enter Terminal and type sudo periodic daily, then sudo periodic weekly then sudo periodic monthly
After these reboot and remember to do them once a week or at the very least once a month. Enjoy the blazing speed.
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