The reason for the slower teleport build speed makes total sense for the team play aspect of the game, I haven’t thought of it that way.
If you’re still taking ideas on what do do in relation to that however, might I suggest some sort of “transit time” that characters undergo when taking the teleporter to compensate? Tracer exhibits sort of what I’m talking about in the sense that she doesn’t immediately pop out of Recall, but there’s a brief period of time she spends unable to be seen/hit as she goes back. This window of time gives the player time to get their bearings in relation to where she’ll wind up, but also gives enemies who’ve paid attention time to position themselves where she’ll end up. In similar fashion, if Symmetra’s teleporter could be deployed instantly but the act of teleporting itself is NOT instant, that could be a useful way to balance it.