I deleted my old answer since it had turned out to be only a temporary fix (if even a fix at all). Today I found something that cleared up most of the slowness for me which I describe in detail in this link:
Yosemite: Accessibility zoom + multiple monitors = poor performance
Somehow my user preference file,
com.apple.sidebarlists.plist, had acquired several keys with very long values (all named
Bookmark); the lengths ranged from about 2.7 MB to 4.3 MB which made the size of the preference file larger than 11 MB. It appears that whenever an operation occurred that needed to access the (sequential) preference data from that file, reading the entire file brought everything to a near standstill until complete. (When I was looking at the file with an editor I experienced the same slowness in moving through the file.) Removing that file has eliminated nearly all the slowness that I was experiencing since switching to Yosemite and continues to do so.
I do think we're dealing with several different issues that are causing Yosemite to slow down and any of us may have one or more of these issues. I'm still seeing long dumps of trackpad gestures in my system log which I had felt was the cause of the slowness but now appears to only slow things down at certain times (as well as cause Firefox's scrolling to fail) after using Firefox for a while; the rest of the machine seems back to it's normal speed.
If anyone finds the same thing and, if so, removes this preference file then it would be good to either validate or invalidate this as a possible cause since the solution is so seemingly elusive so far. This change has made such a dramatic difference in using Yosemite for me that I hope this is a solution for many others suffering from the slow down since upgrading to Yosemite.