Hardware is where it's at for decoding video, I'm afraid. Throwing your CPU at them is a losing battle.

I've got an old media player based on a 300 MHz MIPS CPU and 90 kB of RAM, which plays 1080p videos of almost any codec, bitrate or container with its Sigma decoder chip. MKV Blueray rips? No problem, as long as the bitrate isn't insane or they don't use some weird codec like Hi10P.

The RaspberryPi can play almost all the same 1080p just fine, assuming the I/O bottleneck doesn't get in the way. The Pi 3 can handle 60 mbit bitrate 1080p video no problem. Again, due to hardware support built into the Broadcom SoC.

Same with an old Android box with an Allwinner A10 Chinese ARM ripoff I own: single core, 1 MHz, plays 1080p fine, even across a LAN; the Mali GPU does the hard work, or at least I think it's a Mali.

Software only, an i5 or even a slow i7 might struggle to play the same videos without hardware support. It's becoming ludicrous, because any graphics card or APU within the last decade should be able to play 1080p, and here we are with people running out and buying the best spec Radeons supported, that they'll never use even the basic hw codec support on.