Needed to mount WordPress cache folder into the RAM of one VPS, to get that little bit of extra speed.
To mount it temporary and see how it works for you, you can use:

mount -t tmpfs -o size=64M tmpfs /absolute/path/to/your/folder/

To make it permanent you need to add this in the /etc/fstab file:

tmpfs /absolute/path/to/your/folder tmpfs defaults,size=64M 0 0