nah the code being put in a Transformer or in the disable component field will have the same performance impact. it has to do with how Transformers would be implemented, it would be the same as a function (or JMP/GOTO command) that is or isn't called at the end of any other function/query/ect. if the code loops through a few mil records and changes all of them, it'll be just as slow in a Transformer as it would be in a disabled field.
transformers are nice so you don't have to go looking around for everywhere you stuck JS you duplicated from one place to another when you need to make a change.... instead you can keep the transformer in the Code UI and just reuse that one. this way if you ever need to change it, everywhere that uses it will have the updated code (having to find everywhere you have duplicate code to make 1 change can get mind numbingly annoying especially if you forget about 1 spot lol)