On the server side, it is ycombinator's decision if they can handle the additional cpu cycles. i believe they can, or else they wouldn't have enabled it.
on the client side, I don't think you can really feel the latency. is measurable, of course, but i don't think you can feel it.
And high(er) latency on HN would be a problem because ... ? It's such a highly interactive website? There are billions of high quality images to be loaded? Hundreds of ajax calls going back and forth every second?
It adds half a second or more to initial page load times. For doing any sort of marketing where potential customers are arriving at your landing page, that's huge.
Not necessarily so noticable. I've been running my personal site with https and the effect is fairly minor on the whole in terms of load times. Not zero, but low enough that your average user on the other side of the Atlantic wouldn't notice much of a difference to normal.
Admittedly this is a poor comparison vs a huge site, but then you'll probably find the server processing time is your greater foe.
on the client side, I don't think you can really feel the latency. is measurable, of course, but i don't think you can feel it.