Here’s a conceptual answer.
Images pasted as URLs → loaded from those URLs. How fast they load depends on how fast those servers will serve the images. The images will be served as is, no extra optimizations. I.e. if you’re linking to a 20 megapixel 8 MB photo, that’s what your browser tab will load.
Images uploaded to Coda → will be served through imgix (more on that read here). Imgix is a pretty fast CDN, and the pictures will be served in an optimized and compressed way, so a couple of KBs each regardless of what you uploaded to Coda.
In both cases the images load lazily, i.e. only when a row enters the view when you scroll. So you’ll still experience the same lags and row heights jumping around when you scroll regardless of what method you choose. Uploaded images could be easier on memory use though because of optimizations that linked images don’t have, so scrolling could be a bit smoother.
Neither method affects doc size significantly, but uploaded files will count towards the 1G/5G per doc quota on Free/Pro plans respectively.