How to reduce CPU load when using https?

Question

Apache HTTP Server

Good day!
I decided to move the site to https. All settings were made through ISPmanager Lite 5. OS Debian 8. Apache + Nginx bundle. The problem is that when accessing the site, the load on the processor rises to almost 100%, respectively, the server response time is on average 7-10 seconds. I understand that for a 1 GHz processor this is not quite a normal situation. Tell me, in which direction to dig? Not very strong in server settings.

Answer 1

As it turned out, it was not the reel at all...
The problem was this PHP warning Warning: preg_replace(): Delimiter must not be alphanumeric or backslash and hung out there, and when switching to https, terrible brakes arose. I removed it and the site is working fine again. Thanks to all who responded!