Soldered memory allows higher bus frequency much, much easier. From a high frequency perspective, the slots are a nightmare.
Needless to say it has batshit insane implications for memory bandwidth.
I've got an M1, and the load time for apps is absolutely fucking insane by comparison to my iMac; there's at least one AAA game whose loading time dropped from about 5 minutes on my quad-core intel, to 5 seconds on my mac studio.
There's just a shitload of text-processing and compiling going on any time a large game gets launched. It's been incredibly good for compiling C++ and Node apps, as well.