Room 1 - First half

Cossack 3 is an autoscroller, so there isn't much to explain that isn't in the videos. There are a few notes though.

First, very simply, the screen does not begin scrolling until you move to the right past the spawn point. (?)

Lag reduction strats

One thing to note, is to shoot the 4 turrets when they spawn to avoid lag-generating bullets. After the destroying the fourth turret, you can switch to a weapon to refill, and then (if necessary) to Ring before the next screen.

To be optimal, you want to kill the turrets as soon as possible. You can sort of light mash to make it easier

Turret 1

Turret 2

Turret 3

You can reduce lag even further by standing still until the 2 spark balls on the left side despawns

While possible, you could avoid killing the Turret for the drop on top, but you don't gain any advantage doing so as you should be fine on ammo for most weapons. 

Turret 4

Refill

Switch to Ring Boomerang and pickup this refill. 

If you didnt missed any Ring shots on Mothraya in Cossack 1. You can skip this drop and pick the one later in the second half, you'll save a few frames of weapon refills. 

Jumbig

You'll find a jumbig at the end of this room. People usually tend to stand of the far right edge of the cloud as a visual cue for the slide.

You then need to react to when he jumps. You'll be fine if he jumps high, however low jump can be a bit tricky to time, not a huge deal if you miss it, but you'll loose a bit as you wont able to grab the ladder as early as possible

Low jump example