The 15-91 I had was taken apart and regreased (by my DH). He did another one for a friend, and that one was so much faster! However I still preferred the stop and start of the belt machine for FMQ better than what I got with the gear driven machine (speed not needed for that).
My Silver 15-91 is decent, but on the slow side (it's a 1932 so one of the first 15-91's) though is shows very little sign of wear. My 15-90 is a much later machine, and well broken in. I've found the well used ones often sew much nicer than those that look like they've had little use.