Here’s a few pointers:
If a Part isn’t Anchored it will interact with other Parts and Players, or has its CanCollide on then a player can interact with it. If I have a place with MANY small Parts I’ll turn the Collision off and make sure they’re all Anchored, then put a simple large Transparent CanCollide Part in so players will touch that.
5000 to 15000 Parts shouldn’t be an issue, unless they are really close together and the player is interacting with many of them.
Did you check to see if you have any scripts running while the game is playing, possibly from Plugins that have been copied and corrupted? Sometimes these plugins will add scripts to your game in really odd places, but they still cause issues.
It depends on what you’re replacing with a MeshPart.
If you have a simple shape then a few regular Parts is better than a MeshPart.
If you have a complex shape like a narrow tunnel with lots of holes and bends then a MeshPart may not be rendered physically so that your character will interact with it properly. They may stand on it and be slightly above the surface, or not be able to pass through a small opening.
It’s not a yes or no question, it depends on each and every case and the factors involved.