I solved that issue for me in a different way:
+ right-click on networkmanager to open 'edit connections'
+ edit the network connection of your choice (in doubt edit one after another)
+ set the MTU not to 'automatic' but to '1492' (this usually is the right one, I don't know how to determine the correct one if isn't, maybe somebody else can help here)
+ enjoy speed!
I solved that issue for me in a different way:
+ right-click on networkmanager to open 'edit connections'
+ edit the network connection of your choice (in doubt edit one after another)
+ set the MTU not to 'automatic' but to '1492' (this usually is the right one, I don't know how to determine the correct one if isn't, maybe somebody else can help here)
+ enjoy speed!