Definitely quality, but the quicker you can do it makes you more efficient and more productive, thus, you can then do something else.
In transformation, especially in a large organisation, you have to play the long game. Take people on the journey to ensure change is effectively managed and personnel are supported and communicated to, at all times.
But, if planned properly, you can get quick results too! At the planning stage, aim for some quick wins / low-hanging fruit. This gives you results that your stakeholders and customers can instantly see. They will be encouraged to take the journey with you and have confidence in the process.
Remember, quality always over speed but, with decent planning, make sure you can get some results quickly too.