Soundwave on 31 December 2016
You can just do the basic math.
Tegra X1 full clocked @ 1 GHz = 500 Gigaflop floating point performance (FP32)
Eurogamer is saying
Switch docked is 768 MHz (means 76.8% of the the max clock). 76.8% of 500 Gigaflops = 384 gigaflops
Switch undocked is 307 MHz (means 30.7% of the max clock). 30.7% of 500 Gigaflops = 153.5 gigaflops
