It is much faster if you sync the files to your local file system. I mostly used images, and for large ones (e.g. from high-resolution DSLR cameras) for the first view it takes some time to generate the preview. And once the preview is generated, it depends on your network connection.
I didn’t try the movie viewer a lot. Some codecs are working but there is not such a large support like in some desktop players.
But you can give it a try yourself: demo.nextcloud.com or Demo instances of 9, 10, 11 and 12 now live