Lacie 6big 24TB lacking disk speeds at new M1 iMac

LaCie states: "Up to 1400 MB/s Reads and Writes"

That's the absolute best you can get under the best of conditions. In their case, likely a laboratory type environment. In the real world, published speeds are often quite elusive and hard to duplicate.

This is not the proper venue to get Apple's attention. It's also possible, and quite likely, your setup does not exactly duplicate LaCie's. You can continue to voice your frustration here if you wish, but the better option is to talk to LaCie and Apple and take their advice.

That reasoning also applies to the more technically knowledgeable here; listen to them if they choose to speak. Your situation is you have it working and most likely you just need to tweak your setup somehow. The truth is, if you were to reach 1,400 as you want, the actual real world marginal gains in time to read and write will not feel huge.

Ultimately how you chase this rabbit is your call ... good luck.