It'll generate static worlds, no physics, AI trees etc. I might not even use animations at all. There will be a user-controlled camera. 

I haven't used threeJS before. I'm hesitant because JS is single threaded, limiting how many objects the browser can render vs something like an Unreal app, and I want to make the generated parks very detailed. On the other hand, maybe it'll be fine because the generated worlds will be static. ThreeJS looks like a more convenient dev experience than other webGL frameworks and Unreal. 

So should I keep going with threeJS, turn back to Unreal, or do you recommend something else entirely?