@rmader That reminds me that this seems like a regression with the switch away from the proprietary Broadcom GL stack and the OpenMAX IL/MMAL-based decoders.
With those (even on the RPi 1/2/3!) it was possible to decode and render 1080p30 smoothly by getting EGLImages from the decoder and rendering them via GLES2.
With the new Mesa/VC4 GL stack and the v4l2-based stateful decoder this is only possible by directly rendering the dmabufs via KMS or Wayland instead of going via anything GL-based.
I didn't investigate this further.