Apache2 ruing slow with site on VirtualBox Share

I`m ruing an Ubuntu 16.04 guest on a Ubuntu 14.04 host in virtualbox.

My Virtual box share is slow.

Ob the guest machine, I installed an Apache2 server, and linked a virtual server to a path on a VirtualBox share on the host. This runs really slow (10 second per Wordpress backend load). When I copy it to the guests virtual drive, it only takes 2 seconds.

Why is the virtual box share so slow? And are there possibilitys to increase the performance? Or are there another possibilities to share files between host and guest?