Hello again,
After more fiddling I managed to slightly improve my site without disfiguring it... Unless mistaken, enabling CDN and CloudFlare together is fatal.
Let's compare settings:
DEFAULTS
GTmetrix PageSpeed: 93%
GTmetrix YSlow: 81%
PageSpeed Insights: 90%
YSlow: 78%
MY NEW SETUP
GTmetrix PageSpeed: 94%
GTmetrix YSlow: 85%
PageSpeed Insights: 92%
YSlow: 91%
Browser:
last-modified: ON
expires header: ON
cache control header: ON
entity tag: ON
W3TC header: ON
HTTP gzip: ON
disable cookies: ON
CloudFlare: ON
Domain: phototrope.ca
security: LOW
minification: OFF
rocket: OFF
@ CloudFlare.com:
performance: CDN + FULL OPTIM.
protection: LOW
caching level: AGRESSIVE
Challenge TTL: 1 day
Minimum TTL: 1 month
auto minify: OFF
Rocket: ON
An optimization expert would have probably taken 5 min instead of 5 days to obtain these results... Well, after this ordeal, at least my ignorance is more precise...
Cheers.