Most often (depending on the cycle in the SSD tech generation) larger SSDs are faster (see new PCI4 controllers who need a number of NAND chips for best performance). A 256GB has a high probability of being not the best choice for compilation. A 2018 MacRumors thread shows:

    - 256 SSD 1400 MB/s Write 2200 MB/S Read
    - 512 SSD 1846 MB/s Write 2500 MB/S Read
    - 1T SSD 2500 MB/s Write 2600 MB/S Read
    - 2T SSD 2600 MB/s Write 2700 MB/S Read

So this is mostly with write, the iMac Pro I'm writing this on with 1TB is in line with ~2800 MB/s write speed.

But of course this could be very different with the new iMac which might have a very fast write SSD in the 256 GB model.