The /cache is on your emby server. The roku app requests images at its own resolution from the server, the server on-the-fly creates them and puts them in /cache. If the images already exist, this on-the-fly manipulation is avoided and the /cache image is immediately sent.
The roku has no memory once the app exits. There is no /cache kept to remember later. Once the app restarts the roku has to ask the server for images for its resolution again.
Browsing in the roku app is how you get the server to create a /cache of images in the rokus resolution. So browse into "all movies" now scroll down literally through every single one until they all show up immediately. Now do the same for TV, and every other library you have. This on-the-fly resizing shouldnt be that long as the server creates its /cache for each image the roku requests.
Browsing to prepopulate the image cache using the app/device you want to speed up image loading on is the only way to accomplish this.
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Edited by speechles, 27 July 2017 - 10:04 PM.