Parts lag, but meshes and parts overall have the same performance. They both lag in their own way. I would stick with meshes but also build some stuff out of parts, to have some originality
In short parts and unions generally have more polys, since extra faces aren’t reduced/eradicated in comparison to meshes. As a general tip, use meshed for complex models that are placed more than once in a place (you only need to load the model once in a place). Tip number two would be to try and make the poly count low. Don’t let your goal be 5k just because that’s what roblox accepts. Try and optimize wherever and whenever you have the opportunity. If you are using blender, make heavy use of the “decimate” modifier.
Parts and meshes can really just depend on size and the amount.
Usually meshes take a lot longer to load at first but once loaded a game will usually run smoothly. For parts they will usually load instantly but it depends on how many are in the game. Either way they both have an equal chance to lag so it just depends on what you prefer using.