my specs is 4GB RAM And 4 CPU core because 1 higher traffic is pretty fast to load the number 2 site is ok to load but the last One is hard to Load Sometimes
How to Load Balance 1 VPS Single Server to a 3 Wordpress Website
The server will allocate resources where needed. If there aren't enough resources then things will slow down.
But if only one site has problems then I'd start looking there for problems - bad code, spam traffic, poor caching, dynamic site with spikes of traffic etc.
You really need more information if you want to solve it. Or you could just try throwing a lot more server resources at it.