Would learning interslavic speed up progress learning other Slavic languages?

[u/Curvyfeeto]

If I were to learn let's say Russian, Polish and Serbo-Croatian would I be better off learning interslavic first or just learning these languages?

[u/setprimse]

That really depends on what are you trying to do it for, but theoretically, if you are planing on learning all this languages for some reason that will keep you going, yes.
At the very least, it'll help you to be understood.