Ask HN: How much does the environment you're in contribute to performance?

It's a circle I've been trying to square recently as part of my career. Backstory essay: I'm 6 years into my career. I spent 5 of those in the same company - company A, working up from a Grad engineer to an SDE3, 3 promos, in that company. I had a very few stumbles there and always performed and handled transition pretty well. I jumped ship January of this year over company changes, projects getting cancelled and stopping from a revolving door of Product heads coming in and out. My goal is not to get a senior title but to progress in my knowledge and move on from JS over to a Java focus. I want to build a product or even just some features; I want to code and earn my way to seniority; not start and stop every quarter. I move to the next company, Company B, in March as SDE; everyone else on the team is Senior SDE with 7+ years experience and above: I'm the most junior. Company B is a start up and I'm expected to contribute from the off; build out big features an