Keep in mind that MPEG-1 was designed to give acceptable performance on the computers of the time, so it is a far lower complexity codec than those which came after it, and explains why current hardware can provide acceptable performance with it even with JS overhead. The standard widely-used codecs roughly have a history like this:

    H.261 - first standard DCT-with-motion-compensation codec
    MPEG-1 - B frames, variable rates and sizes
    MPEG-2/H.262 - interlacing, 4:2:2 and 4:4:4 subsampling, other minor features
    H.263 (FLV) - mainly low bitrate improvements, introduction of intra prediction
    MPEG-4 part 2 (DivX/Xvid, etc.) - more prediction modes, some very advanced and little-used features (3D shape coding?)
    H.264 - different transform, even more prediction modes and features
    H.265 - not really familiar enough with this one to say

Also worth noting that this isn't a full MPEG-1 decoder, since it doesn't support B-frames. The justification given in the documentation is "no modern encoder seems to use these by default anyway", but B frames are what gives MPEG-1 a significant compression advantage over its predecessor, H.261; so I'd consider this implementation to be closer to an H.261 with variable frame sizes and framerates, which is useful enough.