Macbook M4 Max to external ssd speed.

Macs don't support USB 3.2 Gen 2x2. When you plug a USB 3.2 Gen 2x2 device into a Mac, the connection speed will fall back to something that both the Mac and the device support (e.g., USB 3.1 Gen 2).

My understanding is that the USB4 specification defines three incompatible "up to 20 Gbps" USB transfer modes. One of them (a new one) is mandatory for USB4 host ports. USB 3.2 Gen 2x2 is optional and has, in effect, been "thrown under the bus." While it may seem a bit odd that your Mac's USB-C ports support Thunderbolt 5 (which can theoretically handle 120 Gb/s of unidirectional video data) and USB4 40 Gbps, but not USB 3.2 Gen 2x2, this combination is, in fact, allowed by standards!

For the fastest connection between an Apple Silicon Mac and an external SSD, you would want a NVMe SSD which has a Thunderbolt 3/4/5 interface or a USB4 40 Gbps interface.

Thanks very much for transcribing all that, but I REALLY wanted a link to the drive- makers specs, because only they include the footnotes like these:

1Performance may vary depending on host configuration. To reach maximum sequential read/write speeds of up to 2,000 MB/s, respectively, the host device and connection cables must support USB 3.2 Gen 2x2 and the UASP mode must be enabled. 

https://www.samsung.com/us/computing/memory-storage/portable-solid-state-drives/portable-ssd-t9-usb-3-2-4tb-black-mu-pg4t0b-am/

your Mac features USB 3 speeds up to 10 M bits/secs, as shown in its posted specs, and the drive speeds you are achieving through this setup approximate 1,000 M Bytes/sec, as expected.

please post. link to the specs of there drive enclosure you are using.