Don't let your slide movement be lazy. Even in legato passages, the slide must move instantly between notes to avoid a "smearing" sound unless specifically marked.
Unlike a saxophone or piano, where a specific fingering or key usually produces a specific note, the trombone relies on a combination of slide position and embouchure tension. When sight-reading, your brain has to process: Where does this note live on the slide? jazz sight reading trombone
Look for notes with an "x" or a small parenthesis. These are felt more than heard. If you don't see them coming, they can trip up your slide timing. Don't let your slide movement be lazy