Hey @AJ_Joe - you are able to manually remove the mixins that are not used. But please be aware. If there is a preset still on the website that uses these, the preset will be damaged.
We did remove the Optimize button moments ago. The respective article has to be updated yet. Thank you for pointing this out to us!
Regarding your fifth and sixth point - the lazy load was activated. Therefore, the images are reloaded. Hence, the delay. We do recommend it is best not to use them in the banner.