The thing that gets rewarded fastest in tech is having an answer. Not necessarily the right answer — having an answer. The room moves toward the person who speaks first with confidence, and the tendency to update on new information gets read as uncertainty, which gets read as weakness.
I've been in this industry long enough to have watched several fast, confident answers cause expensive problems. A quick architectural decision that seemed obvious and turned out to constrain the product for four years. A fast consensus on a technical approach that nobody had actually thought through, which everyone discovered simultaneously six months later when it failed in a way that a little more whiteboard time would have predicted.
The board is partly how I slow myself down. If I have an instinct about something, I draw it before I say it. Drawing is slower than speaking. It forces me to make the implicit explicit — where are the boxes, what are the connections, what am I actually claiming? Half the time the drawing reveals that my instinct was missing something. The other half of the time it makes me more confident, because I've now seen it hold up under the scrutiny of having to draw it.
I've gotten some version of "you think too much" over the years. I take this as a compliment, though it isn't intended as one. The alternative — thinking too little and executing fast — has a long and documented track record of producing things that need to be redone.
The board doesn't make me slow in general. It makes me slow at the right moments — before a decision gets made, when the cost of being wrong is still low. After the decision I can move as fast as anyone. It's the before that matters.