First of all MEC is compatible with PHP 7.0.x – 7.1.x – 5.6.x versions.
With regard to the low speed of your website’s loading:
Deactivate all your plugins event MEC and let the MEC remain active only, set your theme on default, then reload the page, if the issue was solved, then activate your plugins one by one and your theme, refresh your page and check to see which plugin is causing the issue. Please let us know the results.
If the issue was caused by MEC let us know, however, first check whether the configuration of your WP is ok: https://webnus.net/deep-premium-wordpress-theme-documentation/what-are-best-wordpress-php-settings/ Also you need to make sure that cURl is enable in your website (contact your host provider to enable it for you)
If MEC was not faulty contact your host provider for you website’s TTFB : https://en.wikipedia.org/wiki/Time_to_first_byte