Hey guys. My friend and I were experiencing this issue. When I connected to a different router, he and I were finally able to join each other's worlds. Hope this helps.
After doing some testing in a single player world, I have concluded that this only happens on a realm or a server. Again, I know that this is not a connection problem on my side.
Just happened to me while I was trying to speedrun. I was using optifine and shaders.
It doesn't show up in the routers logs. It's not a connection issue, but the type of lag is like desync.
Forgot i was watching this issue. We found out that there was a hacker on our realm who was causing the crashes.