Your live view should be smooth, no buffering, but if there is buffering then it may be your phone, you could try playing the live view on your computer instead...
Reading stored video off the SD card, the camera is a slow SD card reader, you can't expect it to be faster than plugging it into the computer USB2 port and it will be much faster to put a U3 card in a UHS USB3 card reader. Also you must stop it from recording since it is very slow to read and write to the SD card at the same time.
You may also find the 5GHz wifi to be slower than the 2.4GHz, try them both.