Also, "plate mail" went through a lot of developmental stages. Earliest forms - usually called "plate armor", if my vague recollection of something I read once is accurate - were extremely heavy, very limited mobility, if you fell down you might very well not be able to climb to your feet. Got better progressively.
So can't really make generalizations - good to keep that in mind and clarify what you are describing as best you can.
They were never quite that unwieldy, much like the myth about knights needing special platforms to mount their horses. But certainly as steel smiting techniques improved they were able to gain equivalent protection with thinner steel, and they improved the joint mobility as well. This was offset a bit by needing to increase the level of protection as things like crossbows became more powerful.
