E6 in 3 years? Is there such thing as making rank too soon?

For some who are not mature, it can be.

What happened to time rate?

YES, Too fast.
Not enough time in to even get the experience of different units ,Or the different types of people and how to deal with them.
In the Military you are supposed to treat every one the same. We all should know every one is different, And in fact we can not treat every one exactly the same.
What I am trying to say is. There is no way to know how to treat or lead most kinds of people.
Let alone have time to learn how to be a leader and know all there is to know about your job and be good at both.
That tacks time and the most important thing of all experience.
I have seen to many people get promoted because they looked good on paper.

Well I went from private e-1 to sgt e-5 in 8 months and 26 days. I was offered e-6 to re-enlist at the 2 year mark. I got out instead... stupid.reenlisted 6 months later. took me two years to make e-6 again then e7 at 9 years

Honestly? Yes, it can be. Not on a technical and knowledge level, certainly not, but on a maturity level, absolutely. When you gain a new rank, you gain new responsibilities and leadership; maybe roles you are not yet ready for. Going through my years, we were pushed to try and advance in rank as soon as possible. I can't remember if it was because it was an actual rule or just peer pressure. I knew some people who were unsure, but ready but at the same time I knew people who were just not ready, mentally, to take on the role. I understand the need to fill in slots, but we can't sacrifice our quality and hope they'll just fit into it right away.

You know the (Fill in the Blank Army, Air Force, Marines, Navy) doesn’t make mistakes. Right?

Yes.

I do not believe time is as much of a factor as is merit.

SSG E-6. In one year and 11 months. USARVN to Ft Ord, CA

Without knowing the E-6 personally I can not make an honest comment. Is it well above the average, perhaps this person is well above average.

While it was a different time, I was USNR. I got E-2 by taking active duty prior to the one year which was standard. After a few months in Gulfport Chief offered me E-3 because there was a real shortage of knowledgeable people (at this time, 1966, Guys were getting E-4 or above because of trade experience) we called them IPO's (Instant petty officers) Many were good tradesmen, but without experience poor leaders. I was very driven, and actually enjoyed tests, plus one year of experience (apprentice Local 361 NYC) BURPERS gave me E-4 soon after deployment (3/67)I was mess cooking so the crow was VERY welcome :). When we rotated home I had five months left, then almost a month of leave, I could NOT be assigned to a Battalion because I had due to separate 6/68. I had no intention of shipping over so they sent me to Bainbridge, MD in Public Works. Why wouldn't they give me an early out, but you know the USN.....With 6 weeks left they offered me E-5 IF i extended for a year, but I politily thanked the Chief and told him I was set on becoming a PFC (poor F civilian), which I did.