Ok, I have a second story toilet that has been draining really slow for the past 2 months. The water moves from the tank to the bowl at normal flush speed and fills the bowl almost to the top and then about 15 seconds later the water will slowly go down the drain. Tonight, I flushed the toilet just to check it, and it overflowed from the bowl, that was a first and what prompted me to post here.

Here is what I can tell you:
Toilet is on second floor
The other toilet on the second floor is fine
All showers and tubs on second floor are fine
The toilet is 3 years old with no prior issues

Here is what I have done so far based on ideas from the internet.
Check/cleaned all the rim holes
Snaked a coat hanger down the trap to check for clog
Dumped 2 gallons of water into the bowl and it washed into the drain like a normal toilet and apparently that means it is not a clog

I am at the end of my rope and I need help.

Please let me know if you have any ideas.