As Beers as said, make sure both machines are set for the same setting in speedtest, Mbps or Mb/s. What internet speed are you paying for actually? A lot of things can determine the actual download speed difference in devices. What I would do is tether your phone to another device to compare speeds. If its the same then you can most likely assume your phone has something to do with it. And usually the tethered device will not get anywhere near close to what the tethering device is getting.
If your phone truly is getting 30Mb/s then your total bandwidth would be around 240Mbps or a little higher. Go to
www.speedtest.net and do a screenshot of your speedtest for each device and post back here. You may be misinterpreting Mb/s or Mbps. Mbps is megabits per second which would be your total bandwidth from your provider. Mb/s is megabytes per second actual data transfer speed. What you do is take your total bandwidth speed and divide by 8 which would give you your download transfer rate. So if you pay for 300Mbps speed from your provider, your actual data transfer speed would be 300/8 or roughly 37.5 Mb/s.
So you are either having an issue with the different speeds or you have a hardware issue between the phone and the tethered device.