What does this signify

[GreenGecko]

1. It's a SSD VPS.
So what? The whole idea of getting performance out of a VPS is to minimise disk access. All this does is speed up startup time ( Obviously an oversimplification! )

2. Resource utilisation is under 10% for the server.(CPU, RAM and disk)
Good!

3. Caches(server and client side) are properly implemented.
That's a bit questionable... server side caches are how you speed stuff up.

4. The test location is barely 300 miles from the server datacenter.
Good for latency.

This long TTFB time is indicating that the server needs reconfiguring. As you say there are plenty of spare resources, then it's up to you to utilise them to improve things... sitting around not using them is no use to anyone! So look at the performance of MySQL and PHP and what can be done to improve things.

Your DNS provider also seems to be adding to delays - with the test client taking the best part of a second to get an answer. Maybe look at an alternate provider? ( Also, if you're accessing remote services in building your web page, do you have a local DNS server installed ).

As delivery of static resource requires 'zero' server side processing, it's unsurprising that it's much shorter.

There are other things that could be done, but it's the law of diminishing returns...
The ot