I am guessing you are talking about time to first byte (TTFB) which is 900+ms in your case.

DNS caching has nothing to do with it. if you are not using full page cache 900ms time to first byte for magento is normal. here is what you can do to get even better TTFB:

• profile code with magento built in profiler to identify bottlenecks in your code
• install full page cache (by default comes with magento EE)
• go for varnish, HHVM or php7 (with inchoo patch) if you are running VPS
• consider increasing your hosting power (sign up for bigger(faster) plan) or get a VPS or dedicated server and configure it your way

here is an article I wrote on how to optimize magento TTFB

The cache issues in most common where we find (TTFB) in many case, DNS has nothing to do with it here is the some common issues which can be hamper your site.

https://www.cloudways.com/blog/magento-common-issues-their-solutions/

Hope this information help you out.

Here is some article which helps you:https://www.mconnectmedia.com/magento-speed-optimization