Regarding performance - Airbyte is very heavy consisting of multiple JVM containers, nothing you would like to run in a Gitlab workers. Of course I could deploy it persistently somewhere and just deliver a new config there. This is the next step (production-like deployment), where this cons of Airbyte won't be that important. But, from cost perspective, running Airbyte is more expensive.
Personally, I do not have much experience with Fivetran. It is basically too proprietary 😉