Transfer speed

Per Apple specs:

Expansion

Two Thunderbolt / USB 4 ports with support for:

• Thunderbolt 3 (up to 40Gb/s)
• USB 4 (up to 40Gb/s)
• USB 3.1 Gen 2 (up to 10Gb/s)

Search on your "Mac Air (M1)": https://support.apple.com/kb/SP825?locale=en_US

A bit difficult to answer: depends on the external maximum speeds, the cable(s) used, etc. Here are a couple of articles

https://www.iphoneincanada.ca/news/m1-air-benchmark/

https://www.apple.com/newsroom/2021/10/introducing-m1-pro-and-m1-max-the-most-powerful-chips-apple-has-ever-built/

Apple makes no mention in their specs page

https://www.apple.com/macbook-air/specs/

Not forgetting the speed limit on what an older external drive can offer.

This Western Digital My Book drive (2017?) which I have specifies USB 3 data transfer speed which is nominally 5 Gbit/s. I take that to mean it isn't capable of a higher speed even with a 3.1 port on a new Mac.