External Hard Discs Slow on M1 Mini

That sounds rather good for a basic portable drive assuming you mean MB/s. Both manufacturers, Western and Seagate, omit read-write speeds from their data sheets, probably because that’s not their strongest point. I would ignore Blackmagic and continue using. Or shop for newer, faster ones that put some numbers in their specs. Be wary though that most add fine print that make their boasts, essentially useless. All the best!

There are no issues using USB 3 drives on either port. Details, please. What does “noticeably slower” mean? What make/model/capacity drives? Are they for TM backups or regular data? Note that TM runs in the background and does not interfere with normal work.

Yes, by at least x2. Expect at least 200 MB/s, more depending on the file type.

However, they’re noisier, bulkier and need a separate power supply. I had https://storage.toshiba.com/consumer-hdd/external/canvio-for-desktop for years but have since packed it up to the basement and replaced it with a portable. Not because of any performance issue, but to simplify life (discontinuing backups, retiring Time Machine, no user data on internal SSD, moving essential files to the cloud and non-essential to external).

Depends what speed your portable 2.5" drive speed is. There are drives from certain manufactures like WD Gold series that are 3.5" drives that are very fast but as noted are noisier. I have a WD 4TB drive that runs about 200 MB/s vs a normal drive at about 130 MB/s. They do require a powered sock to work

Looks ok for mechanical drive. With the new machines they work best with ssd drives. Since you have the latest Mac you might want to take advantage of the faster speeds available.