Earlier quoted context omitted.
My understanding is that the preferences should not be an opt-out of a default setting per the GPDR, they should be preferences that requested and then saved. So surely the opt-in setting would take just as long as the opt-out setting, wouldn't it?
The opt-in should technically take more time, since you shouldn't be sending PII data before the consent. In the case of opt-out the only single thing that has to happen is setting a local cookie and closing the modal window, which are things that also happen when you accept.
But opt-out should not set anything. I don't know what you mean by "local cookie", a cookie is always sent over the wire by HTTP. If you mean saving to LocalStorage, then I don't think that's allowed either.