There are perfectly fast m.2 modules, that would mean only minor differences in peformance, if any at all. My iMac actually runs perfectly fine on a SATA SSD, though this is definitely slower than the soldered-on SSD of my MB Pro.

Soldering it down doesn't provide any quantifiable performance improvements. It probably allows them to make it smaller / thinner, and less expensive to manufacture. However, now if any one component (memory, CPU, SSD drive) goes bad the entire set must be tossed! When this happens, it will be expensive for the unlucky customer. All so Apple could avoid including a bit more plastic for the component sockets.

Also, SSD drives have a limited number of write cyclew before they stop working, usually some 10's of TBW (terabytes written). For normal users, it will take them a very long time to wear out the drive. Conversely, developers and power user types may find their setups doing 50GB or more of writes per day. So for example, if a drive is rated for 75TBW, a power user could wear it out after only a couple of years.

I have some Samsung SSDs at home where I regularly track and monitor the write usage, and I'm always surprised at how many writes occur on my [often idle] Windows machine. Background OS and application activity is my hypothesis for the cause. The primary windows partition drive will probably need replacing within a year, and will have lasted three or four years.

Quarterly economic performance.