For this the projectile can probably spawn an entity (the crater) with an aura (slow entity in the crater), have you tried that? What would cause performance issue?

I used to think about the craters left by explosions since, to me, they serve as a measure of firepower, but it seems they would cause issues for performance and gameplay.

In the code there is a TODO comment that hints that terrain bonuses already existed in 0AD, but got removed .

Thank you, I'll keep that in mind. I have a type of rocket launcher that can fire anywhere within range, you can simply click on any point, and it will fire regardless of whether there is an enemy there or not.   I really like the small details, like applied forces or weather conditions, that you and others are experimenting with. I hope someone does something with water and terrain as well.

A recent example is @Vantha and I working on improving ranged attacks, so that units can have a range much larger then it's vision and still be able to attack. Perfectly suited for like long-range artillery that needs intel on it's target before firing (like a infantry recon commanding artillery fire, or scout drone...). I actually did have in mind one of @Eilat's post with ICBMs when doing it.