image compression surely will help, but won't improve the base page TTFB. once your page is in cache, your site should fly, really. is the TTFB of the other requests also high? do you see the same issue when selecting another test-location (one near to where the serer is)?
True re image compression, but I'm out of ideas how to improve TTFB. Wordfence's Falcon engine occasionally lifts the grade to a D whereas Fastest Cache drops it to a consistent F but increases the Yslow score in GTmetrix by a point or two over Falcon. The only other thing I can think of to aid the issue is to upgrade the hosting package but I can't really afford that right now.
Don't know if this sounds daft but once you've activated a caching engine how to do you ensure the page is in cache? TTFB of all pages that I've tested on the site seems to be lowly D's or F's if that's what you're asking? When you question whether I see the same issue when selecting another test-location do you mean if I see the same scores if I run the benchmarks from another PC? In terms of being nearer to the server that's apparently based in the Netherlands!
even for the autoptimized .php-files? that's very weird.
I'm not sure, I'm not knowledgeable enough to answer that yet! What I do know is PageSpeed keeps telling me and so does WebPagetest with its D (69%) score. PNG files mainly in the header seem to be the culprit for the lacklustre leverage browser caching score if that helps?
Shane