Recommended disk format for external SSD running as system drive for a 2019 iMac
I'm setting up a new 2019 iMac with an internal Fusion drive. I have an external 1TB Samsung T5 SSD drive that I'd like to use velcroed to the back of the iMac as the main system drive in order to improve performance.
Should I format the SSD to APFS or HFS+?
The iMac has MacOS 10.15 Catalina installed by default.
(I've read some stories of 2 minute wait times after restarts using APFS on external system drives, but I'd like to use the most up to date system if it's right to do so).
Only APFS is supported for running Catalina. There may be some weird workaround for running Catalina on HFS+, but I don't know of any.
Pedro has already correctly answered your question. This answer addresses your statement of improved performance. The internal fusion drive is basically a SATA HDD and a PCIe SDD. While your 1TB Samsung T5 SSD drive will be faster than the internal HDD, it will not be faster than the internal SDD. The reason for this is the T5 uses a USB 3.1 Gen 2 interface through a USB type C connector.
To compete with the PCIe SDD, you would have to step up to a Samsung X5 SSD drive. This has a Thunderbolt 3 interface though a USB type C connector. This Thunderbolt 3 interface is 4 times faster than a USB 3.1 Gen 2 interface. Both interfaces use the same USB type C ports on the back of your 2019 iMac.
I realize the cost of a X5 is significantly higher than a T5. There are cheaper Thunderbolt 3 SDDs, but the lower price usually means less performance. Some comparisons are given below.
$170 Samsung T5 1 TB USB 3.1 Gen 2 SDD with speeds up to 540 MB/s
$200 VisionTek 1 TB Thunderbolt 3 SSD with speeds up to 1800 MB/s
$409 Samsung X5 1 TB Thunderbolt 3 SSD with speeds up to 2800 MB/s read, 2300 MB/s write
The other issue with a USB SSD is whether or not UASP mode can be enabled on a Mac. Note, the link for the T5 has a disclaimer which states the 540 MB/s is dependent on the ability of the computer enable UASP mode. This issue does not seem to occur when a Thunderbolt 3 interface is used.