TTFB How fast it should be about?

TTFB factors

• Geographically relative source and target distance = how close test/visitor is to your server
• DNS, server connection & response time
• MySQL or database backend performance/settings
• Server hardware

but TTFB doesn't mean much - you should care more about initial render time - particularly above fold. Example, check results at Webpagetest.org World Country Flags Demo and TTFB doesn't correlate to what is fastest for page speed.