as the others said, yes turn off cancollide for objects the player won’t be colliding with. to touch on this further if you really want to optimise the most you can, you can turn cancollide for everything and make hitboxes for each part the player touches.
unions have a bad rep for being extremely unstable with excessive use. when publishing a game filled with unions it tends to stutter and lag spike. however, i dont see any issues with it while playtesting.
furthermore unions are great for creating complex objects using much less objects, aswell as merging two parts together so when you have a texture or material over them, it doesn’t create a seam.
conclusion: learn blender
after learning blender, using the mesh optimisation tools plugin for much more streamlined use of collision fidelity manipulation allows you to view the hitbox of all the supported objects and change the collision settings of a certain mesh id. it is a lifesaver.
hope this helps