What exactly does the Project "Client Poll Rate" do and does it apply on project update or restart?

I’ve tried extending this setting out to 30s but it doesn’t seem to have any impact to the client - tags still seem to update as soon as they change.

Also, does this setting change after simply updating a client or do you need to close and re-open a Vision client? (in my testing I tried both but neither yielded a different result)

It is how often a Vision client asks the gateway for any fresh tag information or new messages. A vision client with that setting would update its bound tags every 30 seconds. It might apply to push messages for client updates–not sure. IIRC, values below 100ms are clipped to 100ms. There might be an upper clip, too.