This experiment is incomplete.
Kurt Schmucker says it should work as well as on your computer's hard drive as long as you're not doing too much read and write operations.
Schmucker recommends SSD2GO hard-drives. The SSD2GO PKT XT 1TB (350€) reads up to 1,024 MB/s. At a more affordable price point, is the SAMSUNG 7T 1TB ($160) which reads up to 1,050 MB/s and write up to 1,000 MB/s (on USB 3.2 gen 2 supported devices). The SAMSUNG T7 Touch 1TB ($190) ships with the same read and write capabilities as the T7, except for fingerprint protection—the disk has a fingerprint reader on top and I suspect it lets you unblock the disk only if you verify your fingerprint. At the highest price point—but the fastest speed—is the SAMSUNG X5 1TB ($400) with a read and write performance levels of up to 2,800 MB/s and 2,300 MB/s, respectively, and up to 40 Gb/s data transfer speeds. The tiny size and portability of the ADATA SE730H 512GB ($170) called my attention—you can bring it in your pocket—but it reads at 500 megabytes per second.
If I continue investigating down this path, I'll write down the hard drive I bought, the process to install and move Windows Parallels to the drive, and if it performs well.
If you want to move your Parallels VM to your external drive, follow Kurt Schmucker's guide.