Negating most help!

CRLSUUUU

How do you negate the following sentence?

For most bus drivers, the presence of a supervisor makes their performance slightly worse than it otherwise would be.

In the video for explaining this ac https://7sage.com/lsat_explanations/lsat-63-section-3-question-06/
JY negate it to all bus drivers, presence of a supervisor makes their performance dramatically worse than it otherwise would be..but i thought to negate most is x you say 0-50% is x. or am I wrong?

Thank you!!