Thoughts on a "Shot clock" during combat?

I'm just wondering how people feel about using the shot clock during combat, the pros and cons, success stories and failures, that kind of thing.

I have one player who never pays attention during combat, so every time it's their turn, they either need to be recapped, do something different than what the other players are discussing, and takes a lot of time deciding what they want to do and reading all their spells/feats etc. This tends to snowball as it bogs down combat, and other players follow suit, and then I feel my energy drag as combat continues. Unfortunately, the area they are in at the moment is a huge dungeon crawl, and next session actually begins in combat.

I've already spoken to the player and it hasn't changed anything, their ADHD doesn't help.

So, I'm considering adding a shot clock, so that if the player doesn't determine what their character is doing in time, they automatically take the dodge action and we move on. I also think it would help with urgency in general, but I'm wondering if I'm overlooking any glairing negatives. Thoughts?