Kdenlive Proxy
What is it? A low-res version of your clip to make editing smoother on meh hardware
How to turn it on?
WELL!
1) enable proxies in Settings->Configure Kdenlive
2) enable proxies in the Project->Project Settings for each project
3) Finally(!) the option will not be greyed out. You may right-click the clip and choose 'Proxy Clip'
Next you'll see a little progress bar as the proxy file is rendered and then a 'P' will appear on the clip.
Seems like that could have been more obvious.

@gemlog Slightly off-piste, but can anyone point me to a discussion about Kdenlive's rendering speeds?