What You Could Have Said
Name the ink, not the word. Everyone knows the word slows you down and everyone says it is because reading is automatic. Halfway through, two of your four keys change colour. The words do not move. Watch what happens to the cost.
A word appears in a colour. Press the key for the colour of the ink, never the word. Four keys, shown at the bottom of the screen the whole time as four blocks of colour.
You will do that twice. Halfway through, two of the four keys change to two colours that were never available before, and two colours that were available stop being available. Nothing else changes. The same words appear in the same inks in both halves.
Before the second half opens, the page will print two numbers: what your second half looks like if a word costs you simply because you read it, and what it looks like if a word costs you only when it names a key you are holding. Then it runs the block.
About eleven minutes, ten short rests, nothing recorded, nothing leaves your browser. Use a keyboard if you have one. Taps work and are slower, which costs you nothing here because every number on this page is a difference between two conditions measured the same way.
If you have a colour vision deficiency, the first drill will find it and say so rather than printing a Stroop result on top of it.