Obviously not dismissing your experience, just adding my own : I tried it recently on a big game that was installed on my SO desktop, and it worked great. Just had to activate the feature on both Steam instances, restart Steam, and then I enjoyed a superfast “download” speed, that was mainly bottlenecked by my drive speed and even sometimes by my computer’s ethernet port limit!
Obviously not dismissing your experience, just adding my own : I tried it recently on a big game that was installed on my SO desktop, and it worked great. Just had to activate the feature on both Steam instances, restart Steam, and then I enjoyed a superfast “download” speed, that was mainly bottlenecked by my drive speed and even sometimes by my computer’s ethernet port limit!