Common VPS performance bottlenecks
CPU steal, RAM, disk queues, and chatty apps. Measure before you upgrade.
Slow is a symptom. On NVMe, disk is rarely the first villain unless you are swapping.
CPU: look at steal, iowait, and a single-threaded app pegging one core. RAM: if you swap, upgrade or cut processes. Disk: check whether you are writing logs like a firehose. Network: a 1 Gbps NIC does not help a chatty database 80ms away.
Upgrade Herald to Knight when the graphs say so, not when a plugin's readme asks for 8 GB "just in case."