Regular parts are supposed to be very optimized. Converting a bunch of parts to a mesh its only practical if:
A. There are a lot of unnecessary (internal) faces that players will never see.
B. You’d like to be able to easily resize, place, and maneuver the chunk of parts multiple times elsewhere.
A good amount of parts? Or one extra mesh to load at the beginning of the game. I would say for just a couple conversions, the performance difference is negligible (after initial loading).
At some point, meshes might be the better option for large chunks of parts that could be optimized further. But then again, a lot of unique meshes means more to load on join, so its a bit of a balancing act.
EDIT: When I say converting to a mesh, I mean exporting to blender and cleaning up extraneous faces, and then re-importing. Exported parts will maintain positional data, so re-importing is easy. Rotation and position will be flipped though, so just reverse the values and it will be back where it was initially.