@Ewan99 

I don't know if it has something to do with it, but I read in a post something about an option that reduced the connection speed by half. Maybe that option was activated when you did the update.

https://community.tp-link.com/en/home/forum/topic/111932

"configured in an access point, the rj45 ports drastically reduce the speed to practically the same (347mb), if I leave it configured as a router if they give 100% of the speed through rj45 (600mb)."

I don't know if it will be this, hopefully someone with more knowledge can help you...