Yea, opcache "caches" the compiled PHP code. So the first "page generation" might take ages, because PHP has to load all the .php files and compile them. But an opcache will _keep_ the compiled versions around, so the next time you need to execute the .php files, it can skip the (very expensive) step of compiling them again.

This makes PHP much, much faster. The only time the files are recompiled are if you restart your webserver, or if you modify the .php files.

I'm not talking about static html generation, that's another thing all together.