> these $5 servers are VPS and the resources allocated are guaranteed so...how can neighbors affect the performance of your server

Yes, they're VPSs, but no, a VPS does not provide the level of isolation that you're imagining. RAM is easy to offer guarantees about, CPU can be (but you can often burst over your guaranteed allotment, which means there's often high variability in performance, even if you always get what you're technically paying for), and disk IO is very hard (and you'll notice cheap VPSes don't even try and promise you any specific amount of IOPS).

There's a million links discussing this, google "VPS noisy neighbor" or see, eg:

https://www.liquidweb.com/blog/why-aws-is-bad-for-small-orga...

First: VPS vs. shared hosting doesn't mean (lack of) fixed resource allocation.

Second, even if they allocate fixed resources to your VMs isolation is not that perfect, and you can see performance differences in things like memory access, storage speed, ...

Ignoring overselling for a minute, VPS isolation is not perfect. Some resources, like physical disk and network cards are almost always shared. Others, like CPU, memory bandwidth, etc, while theoretically partitioned, still suffer from some amount of contention.