I have two Slingbox 350s, one is setup with a UK Sky+ HD box. The Sky box has an HDMI output that I feed into an HDFury3 and then into the 350's component inputs. I view this one remotely. If I set the Sky box to output in 1080i then I always get 25fps no matter what player or quality setting. This was my expectation. However, recently I noticed that if I changed the Sky box to output 720p then all players (connected devices, iOS, Mac) would stream at a really smooth 50fps at all except the lowest quality setting. This was a pleasant surprise.
However I recently setup a 350 with an IPTV box that only outputs 720p. The IPTV has an HDMI output that I feed into an HDFury4 and then into the component inputs of the 350. I view this on my LAN. With this I can get 60fps at the Best (HD) setting on the Mac webplayer, although often I have to set it to a lower setting and then back to the highest to make it lock to 60fps. It seems a little reluctant. No other players will stream 60fps except on one occasion last night for some reason they all did without me changing anything. I've not managed to get them to do it again.
Anybody think of a reason why the players when connected to the IPTV are reluctant to stream at the higher frame rate?