I have a detailed question about my completed logic board replacement

Okay, so here's a long stories short: I have this mac (listed) and it worked. I hadn't cleaned it for a looong time, so I opened it up (as I have many times before) and removed the fan to clean the dust bunny build up in front of the heatsink to the fan. As I removed the fan, I accidentally put the spudger underneath the whole fan socket and popped the whole thing off of the logic board. Everything was still in tact, I ordered a new fan connector socket aaaaaand I messed up my soldering very badly. I burnt off the solder plates and melted the new socket. Yeah it was bad. In my defense, I have experience with soldering. I just used a soldering iron that was not fine tipped enough. So then the new logic board was on the way! I purchased the new logic board for $320 on ebay...

http://www.ebay.com/itm/Apple-MacBook-Pr...

It came in and it looked amazing! Like beautiful. I had to transfer the left speaker, mic, magsafe, heatsink, and fan from my old one to my new one. I removed my broken logic board (mind you that my old logic board still works, it is just incapable of a working fan). I removed it perfectly with no problems. All the connections and everything had a successful de-attaching. I followed a very detailed video. With my newly put together logic board, I installed it with all of the connections double checked and neat. After I turned my MacBook on praying for the best, once my laptop finally got to the desktop (this took forever omg) EVERYTHING runs soooo laggy. My mouse moves around like it teleports. It takes about 5 clicks on something to actually click it. Everything runs as if it has 0.1GB of ram on a processor from 1991. I have 16GB of ram installed btw. Again, I double checked all of the connections and installed the logic board correctly! Also, everything works! The keyboard, the keyboard light, all the keys, the mousepad, the screen, the sound, the fan, etc. I'm starting to think it's just the board functioning poorly. I contacted the owner in which I bought it from and he assured me that it was a used OEM fully functional logic board. He said I can always return it for a full refund. He also told me to make sure I reset my SMC and NVRAM before I removed it. I did so and it didn't change a thing. It takes a very long while to boot up and then runs so poorly that it's unusable.

In conclusion, the reason I am here is to see if any of you have ever heard or have run into this problem? Is the logic board faulty? Is there a step I should complete before I remove/return this logic board? Maybe a hard reset on the Mac? Maybe a software/interface mismatch? Any new information will be appreciated. If there is no luck with this, I'm going to reinstall my old logic board and see if it still runs smoothly like it did before I removed my old one. (It ran really smooth, it just couldn't run the fan). I want to do that because I want to see if maybe I messed something up during the uninstall and install. Let me know what you guys think. Endless amount of answers is appreciated! Thank you!

Further specs: Crucial 16GB Ram. 500GB Samsung Evo SSD Running OSX El Capitan 10.11.6. Everything else is standard.