@Patatas 

Hello, I'm afraid the current firmware doesn't allow to be downgraded to the previous version, while please help provide below details, and we will try to fix it on the current firmware, thanks:
1. Who is your ISP and what is the model number of your modem? What kind of connection type on the C2300, Dynamic or PPPoE?
2. Without any other devices connected, connect one computer to the C2300 directly and go to speedtest to run the test for 3 times and show us the screenshots of the test results. BTW, simplify your network layout if there are other routers or RE in your network, just leave the modem and the C2300 on, disconnect other devices and run the speedtest.
And please tell me the wired link speed at that time, how to get that? You can refer to the below link.
https://www.tp-link.com/en/faq-2265.html
3. You can refer to the below instruction to narrow down the issue as well.
https://www.tp-link.com/en/faq-2170.html

4. Check and ensure the QoS setting on the route is disabled. It is suggested to reset the router to factory defaults, reconfigure from scratch to confirm if disabling the QoS doesn't help.