Minimum bit rate in this case won't do anything. As long as you have 6Mbps or above set for 5GHz, or 12Mbps for 2.4GHz, you'll be good.
Is the SSID 5GHz only? Its possible that those two clients that had high ping responses were connecting on the 2.4GHz radio, if your SSID is dual-band that is.
Can you identify any interference nearby?
Are you able to update the wireless driver on the Surface? I recommend the 20.120.x.x package.
Make sure the iPad is also up-to-date, but otherwise no real control of the driver in that device.