Your screenshot suggests you have some sort of image proxy that is fetching the images.
The problem is with that proxy.
Since your image URL to a thumbnail starts image?url= So you have a proxy in the way.
And said proxy seems to be converting to webp instead of using jpg which is what Twitch has the images as on the CDN. So you seem to also be converting the image from jpg to webp which would introduce a delay
304 = Not Modified, so the image should be loaded from the browser cache. not the server.
Not sure what you are asking
Kraken is deprecated for removal in February.
And both kraken and helix have nothing to do with Thumbnails. Other than saying “the thumbnail for a stream is at URL x” and both return the same URL.
They have no bearing on the thumbnail CDN.