hi,
i have had a similar issue with my intel nuc 8i3beh. kept losing both wired and wireless internet speed. i did a bit of research and discovered that there can be an issue with ubuntu 20.04 and the kernel that it uses. check your kernel version. if it is 5.4 or lower update to a newer version of the kernel, i updated to 5.7, and the issue should disappear. check by opening the terminal and typing uname -r.
here is a thread about network speeds, it can be due to a few possible issues and this thread lists all the possible solutions except the one that fixed it for me.
slow network speeds
here is the link to how to update the kernel...
kernel update via terminal
i tried all the other solutions (none of which worked for me) before updating the kernel and ever since the update my network speeds have been perfect.