Since you had the problem prior to the upgrade to TP Link, the problem seems to be somewhere else. Check to see if you have the dropouts when the MAC is attached directly to the modem (bypass...

Try reseting it to the factory defaults. See this link. https://www.router-reset.com/reset-manuals/TP-LINK/Archer-C8-v1x Let us know if it works for you.

If you are on a faster speed tier from your ISP, I would think you would see a difference. For example if you have 150 or less, I would not expect much if any difference.

I have heard that some routers will slow down to the slowest device such as having a 'G' device on your 5 GHz SSID or a device taking most of the bandwith. Try changing the SSID, power the router off...

I have a C8 and it took many tries to get the combination right to connect IPv6 to my ISP (Comcast/Xfinity). What worked for me. Yours may be different: Enable IPv6 is on Get non-temporary IPv6...

Have not seen this before. If it were me, I would try reseting the modem to factory specs and reenter the setup settings. Let us know if that helps.