First of all, optimize storage option starts removing your files from the internal drive when a certain percentage of the drive fills up. For example, if all your files take half of the available drive space, it won’t remove anything.
In any case, the optimization removes the less frequently used files first. On a new machine, there is no way OS can know which file you use more and which one you use less. So my guess is that the OS may be trying to copy all your iCloud files before learning which files are used more frequently than others. Just give it a little time.