Yup. Very slow. There was a split E-4 rank when I was in. Senior Airman for a year, and then Sergeant. Not sure how it’s done, now. I made Sergeant in spite of myself, and got the Hell out after 4 years.
I was A.F., ‘63 - ‘67 and the highest I saw anybody go in 4 yrs. was E-4, with the only exceptions I knew of was the U.S.A.F. band Washington, D.C.
Band members entered the band as E-5s and very rapidly made rank. Not uncommon to see 20 year old E-9s.
I made E-4 in 4 years, while many, many got out after 4 years as E-3s.