I feel like if your work wasn’t good in undergrad, they would’ve told you. You either know what you’re doing, or you don’t — regardless of a speech impediment, and that reflects in your grades, and you passed. Also, I definitely would bring it up with your current advisor, that is 1000% valid, especially if it makes you feel belittled, disrespected, or uncomfortable. You’re not making a bad impression by being honest and asserting your boundaries, that’s a strength, not a weakness. If anything, it should gain you more cred by proving you can make it this far (on your way to a phd!) despite having more odds stacked against you. If you don’t feel comfortable disclosing it in person, maybe send your advisor a text or an email. Maybe even have the school admin do it on your behalf. However you do it, I promise it’ll take the weight off your shoulders. Standing up for myself as a PWS was the best thing learned to do.