Upgraded my SSD, Was it not suppose to keep same read/write rates?

Well, I can tell you what I had thought is not the case here. Apple had used a Toshiba unit which had less performance, I was thinking that was your issue here.

Both are Samsung SSD's and they appear to be within the same series (other than size). Its true the larger flash chips do run a bit slower, I'm not sure if that is the case here. I'll need to do some more digging.

The last piece here was the width of the PCIe lanes you got the info on the 256 unit we didn't get it for the 128 unit.

It's in the SATA/SATA Express section 'Link Width' which is just visible in your first snap for the 256. See if it's the same on the 128 unit. Thats the only thing I can see here that would explain the difference from an interface perspective. That then leaves the firmware within the SSD unit. Check the 'Revision' see if that is the same when you look at the 128 unit.

As to settings sorry nothing here to alter. Unlike HD's which the data can get scattered (fragmented) SSD's don't have this issue as the addressing of the data blocks is the same across the entire storage space of the SSD. Because of this inherent difference we don't need to defragment the drive to regain performance.

The one thing that can effect performance on a SSD is the amount of free space. SSD's like lots of free space so they don't overwear a given block area. I recommend leaving 1/3 free on the smaller drives and the larger 512 and higher you should only need to set aside 1/4. Given the fact you just upgraded I doubt you have even used that much space just keep it in the back of your mind.

You might want to get this utility: DriveDX. It will allow you to see the condition of your SSD and how much of it is worn. Again, I don't see this as the issue here just something useful as you use your SSD to keep an eye on it.