I think it's important to realise from the outset that promotion is not a right, nor are 'best case' timelines something on which to place one's expectations upon.
The branch has been re-invented many times since I first joined it thirty-odd years ago. There have been times of feast and famine, fastracks, jetty bulges, pinch points, you name it.
Promotion is based on several factors beyond just being good at your job. The very process of promotion involves appraisal reports which are at best, subjective - the person writing it could be an admin biff, may not adequately know you or, you may not even like each other. I've seen reports on sixteen ET(ME)'s on one ship claiming they are all above average. There was only sixteen on the ship!
The armed forces are fairly unique in that there is a defned promotional career structure, but the thing one must always remember is that no matter how deserving we may think we are, there are always others who are held in higher esteem - often by dint of the fact the reporting officer is better able to articulate your suitability than perhaps your own boss.
You only have to look at the "how long have you served" thread to realise that over 90% of matelots leave the service without any promotion, at the minimum return of service point.
The tip is simply advance yourself as quickly as possible but be aware it may not always happen as quickly as you would wish for reasons beyond your control. If you bank on about four years at each rate, you are unlikely to be unduly disappointed.