OK, now let's try it with a 5kt loss per corner.

You are flying downwind at 20kts airspeed and 40 kts ground speed. You turn the corner and fly the upline at 15 kts. Power needs to be added to get back up to 20 kts before you go inverted or the model WILL appear to drift downwind tail first. So you turn the top corner but slow to 15kts momentarily until the airspeed gets back up to 20 kts. So it only seems to do a funny wobbly corner before it seemingly hovers in mid air. If you want to cover enough ground to make it actually look sort of square you'll need to throttle up to 40 kts just to produce a 20 kt ground speed and make some headway upwind. So you turn onto the downline with 35 kts of airspeed because you lost 5 in the turn. You throttle back so the prop can slow you down and turn the bottom corner at 20 knots AIRSPEED and then fly away at 35 kts GROUNDSPEED and 15 kts AIRSPEED due to losing a bit in the last turn. And if you're sharp on the controls you'll quickly speed back up to regain the 5 kts so the model doesn't feel sluggish due to the low AIRSPEED even though it's ripping along at what seems like 0.8 Mach.

Your statement about having to speed up to avoid crashing is due to you forgetting that speed in the downline will be retained as you turn the corner. You also did not allow for the wind pushing the model's wing in the up and downlines so that it actually looks like it's moving on a 45 degree angle in the vertical lines.