flying with speed of light, you start breaking the walls, causing you to take damage.
with travelling at speed of light, 300.000m/s, and knowing that every 5m there's a grid, and in average 50% of walls are filled with walls, this means you travelled through roughly 30 THOUSAND walls in one second.
knowing that sandstone has a dencity of roughly 2.5g/cm^3, or 2500kg/m^3, and a wall is 5x4 blocks, then we know a wall hitting you has a weight of 50 tonnes.
50 tonnes hitting you at the speed of light is equal to 15 GIGANEWTON OF ENERGY, and that 30000 TIMES, resulting 450 TERANEWTON OF ENERGY THAT WOULD HIT YOU IN A SECOND,
resulting your death lol, well youre in survival mode,