Is it better to marry someone now or after obtaining citizenship to fasten his green card process?

You should consult with an attorney to see which way is faster. After a full case analysis you should be guided in the right direction for your particular situation.

Yes, it will be faster for the petition to be approved once you are a US citizen.

No. It will be faster to just get your citizenship first and then file for your spouses green card.

Although you can get married at anytime, depending on how long it will take you to get your citizenship, and how much of a hurry you are in, it is probably better and faster to wait until you obtain your citizenship before petitioning for you wife. You can also petition her as a green card holder, and then upgrade after obtaining your citizenship. However, the process could take longer. I suggest that you contact an experienced immigration attorney for a face-to-face consultation and give him/her all of the facts surrounding your situation. He/she would then be in a better position to analyze your case, advise you of your options and make sure that the necessary paperwork and documentation is presented so as not to delay the process.

Petitioning as a citizen will take about 3 months vs. 2,5 years as permanent resident. You may want to get married at this point if there is a chance that you will have been married for two years when your spouse receives his green card as it would be a permanent green card instead of a two year conditional card.

Marry now and apply for the green card for your spouse. Then convert to a US citizen petitioner case when you become a citizen.? Especially if you cannot be a citizen soon.