Play mode lag Unity 2018.4.30f1

So I’m making this tutorial but I keep encountering small lags. The background slides from right to left, and it looks like that: slide smoothly for ~2s, then ~0.1s freeze and so on. Tried changing API preference from direct3d11 to openglcore in the project settings but no visible effect. So what’s the problem then?

Also I’ve tried to build the project, same lags appear too.