Yes, it's normal. While COPS is fast even on a relatively slow machine like a Synology with ARM processor the rendering of thumbnail covers is slow.
If you use a browser cache and do not clean the browser cache to often, luckily the thumbnail covers stay in the cache so the next time you want to look at your book collection the thumbnail covers are served from the browsercache which is considerably faster.
When using the OPDS feature of COPS the speed of the machine/synology is less of an issue as only the thumbnail covers you are actually looking at are rendered.