Why are 1080p videos lagging on my pi3?

I have a raspberry pi 3 with openelec kodi. When I stream YouTube or other videos it's flawless. However when I attach my external HDD to USB and try to stream a 1080p mkv file, the frame rate drops significantly and it's unwatchable. The same issue happens when I plug in a flash drive with the same mkv file. The file runs smoothly on my computer.