I also have fiber. Speeds are around 800+ upload. I use Original quality only.
I did some more testing today. The ATSC1 and ATSC3 channels have roughly the same tuning times when remote, however the ATSC1 channels tune in slightly faster on LAN. Seems like my CBS ATSC3 channel regularly takes the longest to tune out of all on average. Some devices have a hard time playing back the Fox ATSC3 channel smoothly unfortunately. It is choppy and has lots of dropped frames. The Nvidia Shield plays it back fine however.