I understand it to mean that things are now progressing well.

The two ways that people normally cook in their home are with electricity, or gas. I'm no expert but the benefits of gas are that the heat is instant and constant, whereas electric elements take time to warm up and cool down. Gas is therefore considered to be faster and more efficient.

So when someone figuratively says "we're cooking with/on gas now!" it is generally understood to mean that things are progressing, and perhaps quickly.

I notice that it is especially used when perhaps something has had a slow start but now has momentum.

I understand it to mean that things are now progressing well.

The two ways that people normally cook in their home are with electricity, or gas. I'm no expert but the benefits of gas are that the heat is instant and constant, whereas electric elements take time to warm up and cool down. Gas is therefore considered to be faster and more efficient.

So when someone figuratively says "we're cooking with/on gas now!" it is generally understood to mean that things are progressing, and perhaps quickly.

I notice that it is especially used when perhaps something has had a slow start but now has momentum.

(I am a native British English speaker)