Are you using the same ethernet lead and port on your router when using the
T400 as you do when connecting with the Mac. If so this would point to the
problem being with either the T400 or some of the settings on it. If it was
possible for you to try connecting using wifi and your connection was OK
then it would suggest thet the problem was the ethernet port. Another option
would be to try connecting with the ethernet port using a friends internet
connection on their router.
I would suggest you check your connection information see screenshot which
shows mine with the connection over wifi using a liveusb. You could also
check the speed of your dns servers using the dig command e.g.
dig https://trisquel.info
which checks your current settings
;; Query time: 2741 msec
dig 1.1.1.1 https://trisquel.info
which checks using the DNS server 1.1.1.1 you can substitute the DNS server
of your choice and also the website url you want to check
;; Query time: 31 msec
In the attached screenshots the time taken is shown near the end of the
output
I have had issues with DNS servers in the past which made web browsing
extremely slow
