Revision f3e98512-563c-4e76-8143-bd50d8cc625d

I have a 2018 Mac mini which has the same USB and Thunderbolt 3 capabilities as your Mac. I also have a Thunderbolt 3 [Samsung X5][1] SSD and a 10 Gb/s USB [Samsung T7][2] SSD. I will base my answers on what I know about this hardware.

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When the Thunderbolt 3 Samsung X5 is connected to the Mac through a USB Type-C receptacle (jack), System Information displays the following under NVMExpress.

[![NVMExpress][3]][3]

Like the internal drive, the Thunderbolt 3 Samsung X5 appears as a NVMExpress drive, except the drive is external. Note that [TRIM][4] is supported. 

System Information also displays the following under Thunderbolt.

[![Thunderbolt 3][5]][5]

The Thunderbolt 3 Samsung X5 has a read speed of 2800 MB/s. In terms of bits per second, this would be 22.4 Gb/s, since 8 bits = 8 byte. Since the speed shown in the image above is up to 40 Gb/s, one can conclude the read speed of 22.4 Gb/s is not limited by Thunderbolt 3, but rather by the NVMe drive inside the Samsung X5. 

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When the USB Samsung T7 is connected to the Mac through a USB Type-C receptacle, System Information displays the following under USB.

[![Type-C][6]][6]

The data transfer speed is unto 10 Gb/s. This is the maximum speed for USB devices connected a USB Type-C receptacle on both of our Macs. There is no indication of [TRIM][4] support, which may not be necessary if the embedded PCIe NVMe technology is sufficiently fast.

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When the USB Samsung T7 is plugged in a USB Type-A receptacle on the Mac, System Information displays the following under USB.

>Note: The USB Samsung T7 comes a cable for plugging into a USB Type-C receptacle and a different cable for plugging into a USB Type-A receptacle.

[![Type-A][7]][7]

The data transfer speed is unto 5 Gb/s. This is the maximum speed for USB devices connected a USB Type-A receptacle on both of our Macs. Again, there is no indication of [TRIM][4] support.

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Conclusions
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You post contains the following four questions.

 - What is the fastest external storage supported on Intel 2020 27" iMac?

    The Thunderbolt 3 Samsung X5 has a read speed of 2800 MB/s, which is 22.4 Gb/s. This means the data rate between the external drive and the Mac must exceed 22.4 Gb/s, which is faster than the maximum USB data rate of 10 Gb/s offered my your Mac. From this, the following conclusion can be made.

    **>Nothing that USB offers will be faster than what Thunderbolt 3 offers and also be compatible with your Mac.**

 - What does the 2020 Intel iMac 27" support?

    The The USB Type-C receptacles support at least the  following devices.

    Self powered Thunderbolt 1  
    Self powered Thunderbolt 2  
    Thunderbolt 3  
    USB 2.0  
    USB 3.0 (same as USB 3.1 Gen 1 and USB 3.2 Gen 1×1)  
    USB 3.1 Gen 2 (same as USB 3.2 Gen 2×1)

    >Note: Apple's [Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter][8] does not provide power from the Mac to the device.

    The The USB Type-A receptacles support at least the  following devices.

    USB 1.1  
    USB 2.0  
    USB 3.0 (same as USB 3.1 Gen 1 and USB 3.2 Gen 1×1)

 - What does that mean for data transfer speeds?

    The USB Type-C receptacles offer Thunderbolt 3 speeds up 40 Gb/s and USB speeds up to 10 Gb/s.

    The USB Type-A receptacles offer USB speeds up to 5 Gb/s.

 - When buying an external drive what do I need to look out for to make sure it will be compatible with the fastest option?

    Look for a Thunderbolt 3 SSD with high with read/write speeds (in the GB/s range). The SSD should also support [TRIM][4]. You can also look for a Thunderbolt 1 and 2 SSD, but these drives would need to be self powered and would require a [Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter][8]. However, finding a self power Thunderbolt 1 or Thunderbolt 2 SDD at a price lower than Thunderbolt 3 SSD is unlikely.

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Speed Comparisons
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||T5<br>External SSD|T7<br>External SSD|X5<br>External SSD|27" 2020 iMac<br>Internal SSD|27" 2020 iMac<br>Internal SSD|
|-|:-:|:-:|:-:|:-:|:-:|
|Source|[The Verge][9]|[StorageReview][10]|[The Verge][9]|[Tom's Guide][11]|[PC Magazine][11]|
|Capacity|1 TB|2 TB|1 TB|1 TB|1 TB|
|Benchmark<br>Apps|Novabench<br>and<br>Blackmagic|Blackmagic|Novabench<br>and<br>Blackmagic|Blackmagic|Blackmagic|
|Average<br>Read|518 MBps|894 MBps|2410 MBps|2467 MBps|2,427 MBps|
|Average<br>Write|475 MBps|840 MBps|1708 MBps|2757 MBps|2,735 MBps|

Advertised Specifications
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||T5 External SSD|T7 External SSD|X5 External SSD|
|-|:-:|:-:|:-:|
|Source|[Samsung][12]|[Samsung][2]|[Samsung][1]|
|Interface|Up to 10 Gbps<br>(USB 3.2 Gen 2x1)<br>(USB 3.1 Gen 2)|Up to 10 Gbps<br>(USB 3.2 Gen 2x1)<br>(USB 3.1 Gen 2)|Up to 40 Gbps<br>(Thunderbolt 3)|
|Read|Up to<br>540 MBps|Up to<br>1050 MBps|Up to<br>2800 MBps|
|Write|Up to<br>540 MBps|Up to<br>1000 MBps|Up to<br>2300 MBps|

  [1]: https://www.samsung.com/semiconductor/minisite/ssd/product/portable/x5/
  [2]: https://www.samsung.com/semiconductor/minisite/ssd/product/portable/t7/
  [3]: https://i.sstatic.net/5RrB5.png
  [4]: https://en.wikipedia.org/wiki/Trim_(computing)
  [5]: https://i.sstatic.net/dKaXY.png
  [6]: https://i.sstatic.net/evL4n.png
  [7]: https://i.sstatic.net/RtXe3.png
  [8]: https://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter
  [9]: https://www.theverge.com/2019/5/13/18518127/portable-ssd-external-thunderbolt-3-usb-c-samsung-x5-t5-specs-test-how-to
  [10]: https://www.storagereview.com/review/samsung-ssd-t7-review
  [11]: https://www.tomsguide.com/reviews/apple-imac-27-inch-2020
  [12]: https://www.samsung.com/semiconductor/minisite/ssd/product/portable/t5/
  [13]: https://i.sstatic.net/nRJmp.png