Speed vs UI are two different things.
Of course, I could be completely wrong about my interpretation of this if Toyotaro does contradict it in subsequent chapters, in which case I would really dislike it.A way to measure this would be to have UI Goku and Jiren race each other in a 100 meter straight dash. The person who wins has higher pure speed. Whereas, a test for reaction time would be, where UI Goku and Jiren have to press a button when a light turns from red to green. The person who presses the button faster, has higher reaction time. So in these two cases, I'm saying Jiren wins the race but UI Goku presses the button faster. They are two completely different things. This is also why Flash, one of the fastest characters in the DC universe, would still lose a fight versus Slade who can predict where Flash will be, trip him, and win the fight.