Hey there,
the march-speed boosters indeed work for your hero , your troops, and therefor for your marches (hero + troops).
The reason you see troops rushing fast across the map is these players do not actually send troops or marches, but rather convoys (these horse trailers that send rescources). These look like troops to other accounts, i dont know why. Only you can see "your horses" ^^ And if others send "their horses", they, to you, look like ultra fast troops.
The reason you might see ppl rush in with their marches when attacking other players is they may be burning a couple of "Marching speed-ups" of 25% or 50%. You can see what i mean, when you click "Boost" on an active march of yours. ;)
Since ppl often used the aforementiond speed boosts, players got in the habit of re-shielding fast.
Start an attack > a pop up will tell you that you will lose your shield when clicking "send" > click "send" > immediatly go to the menue and click on "apply" for another shield. (On PC the way i do it is clicking "i", then "Military" and then "apply" on one of my 8hrs shields.)