In general, if something runs slowly at high resolution but smoothly at lower resolution, that’s a bottleneck with your GPU, so multi-threading with your CPU cores wouldn’t have any effect.
You can scroll through the Defold showcase to see what’s been made with it. I think both Fates of Ort and Family Age are pretty big (and Family Age is for mobile…). Defold is pretty fast as modern game engines go, but comparing it with a game engine from the late 90s…who knows. There’s going to be a big difference in their features and how they work under the hood.
In the end, there’s no way to know if one engine is faster than another without doing identical performance tests on each one, with the same hardware. Each one could be faster or slower at different things. Comparing similar games isn’t very relevant either, since the final performance is at least as much determined by how the developer made the game, as it is by the underlying engine.