I want to make a custom water friction field, here is my code:

let waterResistanceField = SCNPhysicsField.customField { (position, velocity, mass, charge, time) -> SCNVector3 in
    let waterDensity: Float = 1000.0
    let speed = velocity
    let dragCoefficient: Float = 0.2
    let dragForce: SCNVector3 = -dragCoefficient * waterDensity * speed * speed * velocity
    return dragForce
}

And I customed some operation of SCNVector3

extension SCNVector3 {
    static func -(lhs: SCNVector3, rhs: SCNVector3) -> SCNVector3 {
        SCNVector3(lhs.x - rhs.x, lhs.y - rhs.y, lhs.z - rhs.z)
    }
    static func +(lhs: SCNVector3, rhs: SCNVector3) -> SCNVector3 {
        SCNVector3(lhs.x + rhs.x, lhs.y + rhs.y, lhs.z + rhs.z)
    }
    static func *(lhs: CGFloat, rhs: SCNVector3) -> SCNVector3 {
        let c = Float(lhs)
        return SCNVector3(c * rhs.x, c * rhs.y, c * rhs.z)
    }
}

But I get an warning in the first line of the first code: The compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions.

How can I solve it?

let dragForce: SCNVector3 = -dragCoefficient * waterDensity * speed * speed * velocity

Subbing in the types of your variables, this is:

SCNVector3 = Float * Float * SCNVector3 * SCNVector3 * SCNVector3

Now, granted, you made a custom operator *(lhs: CGFloat, rhs: SCNVector3) but that takes a CGFloat, which is not interchangeable with a Float.

I can't say for certain that's your problem, but I think that's probably your problem.