Woocommerce slow checkout

Woocommerce checkout is very slow after payment.

Need to wait about 10-15 seconds.

Maybe it's something with Cache. We installed W3 Total Cache and WP Super Cache before, but this will not fix the issue.

I can give someone access to backend, ftp and give him access to test the system. When we know what's the problem, we can plan a fix for it (need to be done carefull because website is in production).