Probably due to the lack of an SSD.
IIRC, the 2011 is the last non-slim version of the iMac and as such can actually be opened relatively easy (well, I haven't dared even with my 2008 - but might do once the HD gives in). So, with a little investment in time and money (SSD, tools-kit from ifixit), you could significantly increase its responsiveness.
CPU-wise, it seems to be a bit faster (or on par) with my 2012 Mini, which is equipped with an SSD.
And I can't really complain about its responsiveness.
Nobody knows if and when the next update for the iMacs comes through. And then it remains to be seen what exactly the update consists of.
The 27" models already offer Skylake chipsets and CPUs, I would be very surprised to see the successor (Kaby Lake, apparently scheduled for H2 2016) to show up in an Apple product any time before late 2016.
I would also be surprised to see any significant refresh before "back to school".
I actually hope they'll release updated notebooks in Q1, finally. The Iris Pro chipsets are supposed(!) to be available in Q1...