Have you installed it on both ends? Btw, file transfers by default go from client to client, server is no participating. So it is actually how fast the link is between these two clients.
As it is not AN official release, i'm kind of wary to call it "stable" Though official release on this site doesn't mean much in the stability field. Sometimes another or even two more releases are needed to fix bugs in the official release. There are no testing team or Q&A here. Usually developers pushing the update make a decision, that it is stable enough. But when more users start to use it, problems occur. Current Spark build is stable in my opinion. Around 200 users are using it in my network daily. Yes, it can lockup sometimes, but so does 2.6.3.