For the CPU, you can't really know how much it's shared with other VPS.
The RAM is the only part of a VPS which is not shared, so more you have RAM, better it is.
For the disks, choose always SSD disks, and avoid VPS with any other kind of storage.
For the configuration, for me Plesk is a must for performance, and it provide you all the features to have a fast and secured website.
The fastest VPS will have enough CPU to not have slow down (very hard to know before ordering), a good amount of RAM (4GB - 6GB - 8GB), and SSD storage.
But even with that, from a package to another, you will not have the same performance. it's why it's pretty hard to choose an hosting provider.