Unity H4xx (acilAddOns.dll - CollectionMarshal for Unity)

CollectionMarshal.AsSpan now exists in Unity for ultra-fast List iterations ...

Make sure these DLLs go in a folder named "Plugins" somewhere in your asset folder. Just drag and drop them there from the zip file and Unity will pick them up and reload. Now you're good to go.

Add this reference to use CollectionMarhsal directly:
using System.Runtime.Interops Services;

There is also a convenient extension method for Lists that's as simple as:
var span = list.AsSpan();

Let me know how things go! Don't get a speeding ticket!