SageMath takes 3s to load for me (sometimes a lot more if my systems administrator has updated it recently, which seems to happen a lot).

I therefore use Pari/GP for my personal choice. I like the symbolic nature of Pari/GP over Sage or Julia, even though I spend nearly all of my life programming in Julia, and love the language a lot.

However, for computations of trivial things that I want to run really fast, I put them in a function in Julia (so that they are jit-compiled), since nothing will come close to that, by some orders of magnitude.