…which is why you don't go out and start throwing parts at a problem.
I have a neighbor with a 2009 MacBook, 4G RAM, and an internal HDD and it's running Yosemite on it just fine. That system is probably about 1/2 as fast as yours, maybe even slower. What I did notice was that their's was bottlenecking a little and had a few lockups on 10.10.3 so I installed 10.10.4 on it and that cleared it up. That's how I found out about 10.10.4 - I just figured what could they lose? They were already thinking about wiping their drive and putting Mavericks back on anyway.
It's still a beta release, but the improvement was noticeable. Even then though, I wouldn't say it was like a night and day change. Yosemite should run just a little slower than Mavericks.
I don't remember if you've tried the SMC reset or NVRAM resets, so here are links to them anyway:
https://support.apple.com/en-us/HT201295 (SMC reset)
https://support.apple.com/en-us/HT204063 (NVRAM reset)
NVRAM reset probably isn't needed but SMC settings can hit performance.
Other things you might try or ask:
1. Are you running anti-virus software? If so, disable it.
2. Have you run any "clean up" utitilies? These can be notorious for deleting important files.
3. Are you using wireless? If so try hard wired ethernet because there were bugs with wi-fi.
4. Have you tried booting into safe mode? If the system runs normally in safe boot, there's something being loaded, either third party or some driver that's now locking up the system. Camera drivers, for example, may be causing the kernel to keep re-loading stuff it can't load in a loop.
There
are known problems with wireless on Yosemite. If you're using wireless, disable it and use a hardwired connection. I don't use wireless myself because I have a gigabyte ethernet connection and want the speed.
That's all I can think of for now, and I hope it helps.