Thunderbolt 3 bandwidth distribution on new iMac

The truth is complicated, poorly documented, and changes from computer to computer depending on exactly how the manufacturer wired things.

On Apple desktops (and 15" MBP) each pair of ports is driven by a shared thunderbolt controller and provided PCIe 3.0x4 (32Gbps) for data plus up to (2? 4?) streams of Display Port 1.2 for video. While thunderbolt is capable of providing up to 40Gbps of combined video+data, the data component seems to be throttled to 22Gbps maximum to guarantee a 10Gbps minimum reserve for the 2nd port.

So yes, the 15" Macbook Pro and iMac Pro are indeed stronger with two independent port pairs (but still can't push full 40Gbps on every port simultaneously)

On the older 2016-2017 dual-core 13" Macbook Pro touchbar both controllers reportedly had independent displayport channels but shared the same PCIe 3.0x4 data link by way of the internal PCH-U (Platform Controller Hub), making them potentially worse than an iMac for pure data transfer despite having twice the TB-3 ports. The new quad-core 13"s have more PCIe lanes and might be wired differently.

its not the ports but how many thunderbolt buses. Yours has one therefore two ports to be SHARED., I like three buses and six ports found on the 2013 Mac Pro. You should not daisy chain Docks.