Do terrains use more resources than tilesets for collision?

This has been something that has been of interest of me, since I've been using terrains for collision quite a bit more. As it allows for more organic collision than having everything be based off the tile presets you've made. This also means I can use less tilesets and make stuff mostly with actors on layers than out of tiles. Thus, making my workflow less limited.

So my question is: Is a map using mostly terrains going to have a worse impact on the game's resource use than basic tileset collision? (As in, do tilesets use less CPU).

This is something I'm worried about. As small scenes probably have no issue running right with terrain-based collision, but I imagine big scenes will just flat-our crash or lag.

I don't have any reason to believe so, although I've never sat down and thoroughly looked at the differences between terrains and tilesets. Lag is usually the result of too many colliding bodies. There's more room for actors in a big scene, and so more potential for a problem. But the problem can be avoided by careful management of active actors (i.e. keep a watch on which ones are flagged "always active").

No. The terrain collision gets meshed in with the tile collision. In the end, it all becomes one giant collision mesh. You can see for yourself by enabling debug drawing and running your game.