So Nintendo Life made an article on how to speed up your internet. But be very careful with this advice! In many cases, yes it can speed up your connection. But it can also slow down your connection.
MTU is basically how big “packets” can be when sending around the Internet. Bigger is often better for downloads, but there are a couple of problems that can crop up if the MTU is too big: Packets can be split, or packets can be dropped.
Packets can be split (or worse, dropped) when whoever you are communicating with has a lower MTU than you have. This is especially common with DSL connections, which often have an MTU of 1492. If packets are constantly getting split (as would be the case with a download), you will actually get a drop in performance, rather than a boost.
Packets can be dropped when you are on a poor connection, and if a lot of packets are being dropped, a higher MTU will mean you’re dropping larger packets and potentially losing more bandwidth.
This is likely why the Switch defaults to 1400, because it is a much safer number and ensures you’ll almost never suffer from a loss of bandwidth due to split packets.
That’s not to say you should never increase the MTU: If you’re at home you know you have a solid connection and your ISP’s MTU isn’t likely to change, so go ahead and try to bump up the MTU. But it’s unlikely to be worth it to change the MTU on public WiFi.