I appreciate if anyone can give clues on this slow toilet flush problem.

When I turn the flush lever the tank is getting emptied filling the pot almost to the brink, then the pot gets flushed very slow. Now the water level in the pot is much lower than what it used to be when the flush worked normal. When the water in the pot reaches the lowest it makes sounds releasing some air bubbles. Looks like an issue with the syphon effet here. I tried using plunger and holding the flush lever for 15 sec but neither did help. I poked the flush hole and the small hole in the opposite side with a hanger and did not help either. Not sure if there is a clog deep inside but it is actually flushing (very slow though) such that the water level in the pot is lower than it used to be. Any suggetions are appreciated.

Thanks in advance.

This is a good indication that something is in the trap area of your commode, It could be something like a comb, or even a Q-tip. If you had a closet auger you could run it through the commode to see if you get anything out.

One check to see if it is something in the bowl is to wad up a hand full of paper and place it up into the part of the commode that the water goes out of. Then flush it and watch the water level, if it rises and then goes down slow like before that probably means that there is a blockage in the trap.
If the water lever rises and then goes out with a swoosh, that could mean that you have a mineral buildup in the trap and it need to be cleaned with muriatic acid.

I have done a service call twice this year on this very thing and both times it was the head of a broken toilet bowl brush. They both were a pain to get out but I bend the tip of my auger out a little to make a small finger to grab clogs.
Like letmetellu said, something is in the trap of the bowl.

As a rule Letmetellu gives out top notch advice but this time he's dropped the ball with, "If the water lever rises and then goes out with a swoosh, that could mean that you have a mineral buildup in the trap and it need to be cleaned with muriatic acid."
DO NOT USE MURIATIC IN A ENCLOSED AREA! Such as a bathroom. My lungs are permanently scarred from taking limed up toilet bowls out in the alley behind my dads shop and boiling out the lime in the bowl with muriatic acid. I lost a good friend when he succumbed to the fumes of "Blast Out", a sulfuric acid based drain cleaner he was using in a customers bathroom.
Using acid in your drainage is not customer friendly. It's dangerous stuff.
On the other hand he gave you very good advice when he advised you to check for a clog by flushing down a couple of large wads of TP.
If you find that the trap's clear then click on back. I just might have a solution for you. Regards, Tom

I actually am strongly against chemicals in a toilet. It has been my experience that chemicals strip the gloss off the toilet and creates somewhat of a flow restriction. It also makes a fume cloud that irritates the heck out of my lungs. I have used chemicals to clear a urinal, but I don't like that either.

So what is your answer,to change the toilet?