HTTPS is not computationally expensive with semi modern hardware, the problem is latency or delay due to the the back and forth handshake process for authentication. If you have http keep alive on this only happens once though.
Not using https for your entire session enable's man in the middle (eg: when using using cafe, hotel or public wireless) cookie jacking, remember Firesheep! This is why banking sites and even gmail use https the whole time.
yep, MITM attacks are possible in that situation... but are they a risk for geekzone users on the geekzone website?
https also doesnt cache well, so that can also add to the slowdown and erode the user experience.