Strange Question. How is the speed of the eSATA cable in Time Capsule?

So I am thinking maybe is the eSATA cable or the processor in TC was limited?

I was sitting trying to think.. what does he mean eSATA.. No it is just standard internal SATA..

And it has nothing to do with the issue. (Although i think it is SATA 2 not SATA3). Whatever it is slow.. and you are quite correct.

I have an time capsule 2TB, bought at 2013.

That is the changeover year.. so I am not sure if you have the flat pancake model.. A1409

Or the newer tower AC model, A1470.

Regardless both have limitations caused by processor.

However, when I copy or read files from TC, the speed limited was around 30MB/S.

SMB is slow in Sierra unless you turn off signing.

See Turn off packet signing for SMB 2 and SMB 3 connections - Apple Support

I see other people do get faster speeds but my A1470 (or earlier) models never seem to get much over about 40MB/s

(Using B for Byte and b for bits which you have observed).

Certainly it was much much worse with SMB signing turned on.

If it's true, I will not upgrade it. and just buy an NAS..

If you are going to the expense of buying an 8TB drive.. buy a NAS.

I run Synology and recommend them for Mac network. The data performance with any of the decent models is hugely better than TC. It comes out a lot more expensive but it also gives you proper backup.. The Apple TC is a network router with a hard disk inside.. it was never designed as a NAS or file server.

if the TC case can handle full speed from any kind of hard drive..

It is processor bound.. and likely the SATA 2 has some effect.

You could replace the mechanical hard disk with SSD and the speed of the TC file transfer would not change at all.

If you want faster speeds you MUST spend some money.