The simple answer is no It won't slow down traffic for other users.
Your intuition is correct, Port-Forwarding to a game console or PC can drastically improve the user experience in real-time multiplayer games.
PlayStation 4 needs these ports forwarded to have the best online experience.
(Tcp ports) 80, 443, 1935, & 3478 to 3480
(UDP ports) 3478-3479
Xbox ONE needs these ports forwarded, Port 80 (TCP), Port 88 (UDP), Port 3074 (UDP and TCP), Port 53 (UDP and TCP), Port 500 (UDP), Port 3544 (UDP)Port 4500 (UDP)
In addition to this, if you look in the manual for the game you will find ports that need to be forwarded for each specific game.
PC games will be game specific ports. Check in the game manual or search online.
You can think of game traffic like VOIP traffic, low bandwidth, but extremely susceptible to latency & jitter.
If the router has QOS as a feature, you can additionally add these ports in as priority over other devices traffic on the network. This will make sure other users downloads/streaming etc will not interfere with the games you are playing.
Hope this helps.
Additionally, it's the overall traffic that matters, not a specific port. – None – 2019-11-30T18:31:26.417
@Tim_Stewart True, I don't. But your own answer says the same (at least what I intended to say). – None – 2019-11-30T19:37:13.400