Problem with Mesh Collider and Raycasting

So basically for fun, I'm making a Roblox parody called boblok. Movement and online multiplayer (I'm using Photon PUN 2) work perfectly fine on simple colliders, but when I try to use mesh collider on my custom terrain I made in blender, something weird happens. So first of all, Cinemachine FreeLook Camera Collider is really jaggy, and when it collides with terrain it at first just kinda doesn't know what to do so teleports around, and then it is really laggy afterwards. And, worst of all, when the player jumps once, everything is fine, but if he holds spacebar (which I added as a feature, so you can hold and it will jump every time it hits the ground), it slowly flies into the sky, but then stops, and detects something underneath it as ground, which means you can basically float. Here's a video of my problem:

https://drive.google.com/file/d/1Z58QeL0vDQY_pYPitcPcNmloOzMyCUkh/view?usp=sharing