Allow me to correct myself. It was the lack of optional unwrapping 2 lines further down which caused it.
After I unwrapped the rightView and leftView properly, the compile time for that method was dropped by 99.3%. It’s safe to say that I would never have discovered this on my own.
33.2ms CMExpandingTextField.swift:18:19 @objc override func intrinsicContentSize() -> CGSize