Q: Hugely frustrating problems on Mac post osx yosemite

Restore the bootable backup/clone or Time Machine backup of your previously good OS.

27" i7 iMac (Mid 2011) refurb, OS X Yo (10.10), Mavs, ML & SL, G4 450 MP w/10.5 & 9.2.2

Activity Monitor - Mavericks  also Yosemite

Activity Monitor in Mavericks has significant changes

Performance Guide

Why is my computer slow

Why your Mac runs slower than it should

Slow Mac After Mavericks

Things you can do to resolve slowdowns  see post by Kappy

First thing to do is check out CONSOLE.

Usually if you are "lag" it means console is pretty busy. 

I found that a number of my previous "widget" / additional apps no longer work under Yosemite.

- Diary program Chronories for example, was just filling screeds of console pages with alerts and warnings.

- Chronories makers state

Apple implemented some major modifications to the way applications are allowed to interact or access data from different applications. Unfortunately, Chronories is one of the applications that are severely affected by these decisions. While technologies like "Sandboxing" improve the general security of the whole system, several applications can no longer be maintained as before. Even updates to the existing app are nearly impossible because Sandboxing is required for any further update. This is we had to stop selling Chronories... although we really liked it a lot."

Sure enough I removed everything to do with Chronories and ... lag decreased... only a few more Apps to go.

In this case a real shame as I liked using Chronories.

They can recompile it to work in Mavericks and Yosemite, but to that, they would have to make it an universal app and bypass the sandboxing issues which they couldn't sell via the MAS. Carbon Copy Cloner and others do that.

Please read this whole message before doing anything.

This procedure is a diagnostic test. It’s unlikely to solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.

The purpose of the test is to determine whether the problem is caused by third-party software that loads automatically at startup or login, by a peripheral device, by a font conflict, or by corruption of the file system or of certain system caches.

Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards, if applicable. Start up in safe mode and log in to the account with the problem. You must hold down the shift key twice: once when you turn on the computer, and again when you log in.

Note: If FileVault is enabled in OS X 10.9 or earlier, or if a firmware password is set, or if the startup volume is a software RAID, you can’t do this. Ask for further instructions.

Safe mode is much slower to start up and run than normal, with limited graphics performance, and some things won’t work at all, including sound output and Wi-Fi on certain models. The next normal startup may also be somewhat slow.

The login screen appears even if you usually login automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.

Test while in safe mode. Same problem?

After testing, restart as usual (not in safe mode) and verify that you still have the problem. Post the results of the test.