Just a reminder to Apple that this issue is NOT resolved in iPadOS 15.1 Beta 2.
VILLMER
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I believe I have found the solution for HLS:
In Safari > Advanced > Experimental WebKit Features, enable the following
GPU Process: Canvas Rendering
GPU Process: Media
GPU Process: Video Capture
GPU Process: WebGL
WebGL via Metal
Note: Unfortunately, this does not solve loading standard mp4 files in Safari. The load time is still over 10+ seconds before visually rendering