I just started fusing flint2 few days ago as a Wireguard client, and noticed constant buffering when watching stuff on my chromecast. Changing MTU to 1440 fixed my issue.
Nah, not that easy: Wireguard Optimal MTU · GitHub
1440 is mostly too much.
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Tbh I don’t really understand MTU. Someone might find this post useful in the future. I have tried lower numbers and they didn’t help with my issue.
You can’t just try some numbers - MTU needs to be calculated. 
When you use vpn, you will have lower mtu value. The default mtu for wireguard on the router is 1380 (or 1360) which should be compatible with the server side.
But I assume that your chromecast is using a fixed mtu so you have to increase the mtu of the wireguard.
But if you can find a way to lower the mtu of chromecast that will be better.
check some of the posts here
https://www.reddit.com/r/openwrt/comments/lv1rgx/androidchromecastvpn_wont_work_unless_i_set_wan/
https://www.reddit.com/r/Chromecast/comments/bzfx5e/frozen_image_with_sound_chrome_to_chromecast_fix/
The default MTU on my X3000 and MT2500 are both 1420.
Actually I spoke too soon. I’m still experiencing stuttering problem on Chromecast.
I have a WireGuard server in Japan that I’m connecting from the U.S. I use WireGuard on the router so I can watch NHK+ on the Chromecast. Before switching to flint2, I was using R7000P running dd-wrt that I had zero issue with. Now with flint2 it’s constantly buffering while watching stuff on Chromecast even with youtube videos.
I would love to keep the flint2 cause it seems to be a solid device other than the issue I mentioned, but at the same time I’m super confused why this is happening.
make sure you use correct dns in WG client file
I am using 1380 MTU on a slow 3G/WCDMA wireguard client connection on my X3000.
So far so good.
At the default 1420 setting I was experiencing extreme buffering.
I changed my Wireguard VPN client mtu to 1280. Much improved performance.
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