Can You Move in Medieval Armour? An Experiment in Mythbusting - Medievalists.net
Many believe the myth that medieval armour was hard to move in. However, with Boucicaut as an example, our video proves that late medieval armour allowed for surprising mobility.
Except of course that English knights didn't wear plate in the 12th century and longbows would have been very efficient against the maille that they would have worn.
I'm not debating the historicism of Ivanhoe, but rather the origin of the idea that plate armor (specifically high medieval) was cumbersome and the knights were unable to move well in it.
And depending on the range, and draw weight of the bow, mail is somewhat effective. Plenty of Youtube vids on this.
Anyway, you may be right, it could be 19th century fiction...but the origin came from a number of sources high disconnected with both the time period and martialism.