EE-SX1140 datasheet confusion
VCC = 5 V, RL = 100 Ω, IL = 5 mA

Read what it says; load current is 5 mA. This means that the load voltage is 0.5 volts and not 5 volts. 20 mA is not present in this formula. If you want high speed you sacrifice output voltage level because the load resistance has to be naturally smaller for high-speed: -