The reason Burma can see them fighting at times is purely because sometimes fight canon is taken out the window in favor of PIS.
Same reason every character Superman fights that has him physically bested isn't shaking the planet with their hits or even the city.
Burma has struggled to see them during the namek saga and before then. Hell every Z-Fighter was struggling to keep up with Goku's movements in the Battle of the Gods movie when he became a Super Saiyan God and fought Beerus.
Their energy output at this point is to much for everyone else.
The only way anyone has a shot putting them down is if the DB team gets stupid or they get dropped by TP..
Also Kamehameha seems slow sure but the timeframe isn't the actual minutes were viewing the Kamehameha being fired and charged up. Anime kind of does that. Example is Ichigo when he first started controlling his hollowfication be could only hold it for 10 seconds. He had a fight despite this and under this timeframe that was about 1-2 episodes.
Same reason every character Superman fights that has him physically bested isn't shaking the planet with their hits or even the city.