I worked for Sandia. Pay is pretty good by almost any standards except FAANG. The glory days where every staff member got a real office with a real door are over (shared offices are the norm) but it's still a pretty decent work environment.

Things don't move fast, as another commenter said. In my area of work, projects tended to last 1-3 years and you'd be on several projects at any given time. In general, it is ICs rather than managers who run the projects. Your manager might say "Bob over in 9876 has a neat project that could use somebody like you, send him an email if you're interested".

You have to acknowledge that the core mission of the DOE National Labs is nuclear weapons. You might not ever come in contact with the mission, but it is there. They have strong HPC programs--because HPC as we know it is basically driven by the need to simulate nuclear weapons. Some people have moral objections to this, and that's fine!

I thought it was a good place to work, all in all.

Edit: I'd like to stress that probably the biggest advantage of the labs is the opportunity for self-directed work. If you can convince somebody (external sponsors, internal R&D funding committees) to give you money, you can work on just about anything. If you can't get funding of your own, you are still more or less able to choose what you work on.

Your work environment will depend highly on which group you're in. Some groups look like a university department without the students: you work in the unclassified area, you publish papers, you can even open-source software (with some effort). Other people spend their whole day in a windowless SCIF working on very sensitive stuff which they can never, ever discuss outside of a SCIF -- but while their public visibility is nil, their impact is arguably greater.