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I know nPlayer is able to tap into native HEVC hardware decoding for the MKV container format. Not sure if it’s supported in all iPad Pros though.
 
Anything with Apple A9 or higher will do it (ergo iPad 5th gen or faster) so all the Pros should be capable.

At high bitrates though (e.g. 100+Mbps 4K UHD Blu-ray), it's the wifi connection that'll likely pose an issue.
 
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