8 GB is the reason, especially if you're using Raw files. Adobe will rip through RAM on my Windows machine with a dedicated GPU memory. I've seen it eclipse 20 GB of RAM from time to time (mostly because it's available it will cache images).
Apples M series chipsets are very efficient with everything being contained on a single die. With that, RAM is a shared resource for the CPU and GPU. Since Lightroom accelerated certain tasking via GPU (like heal or masking) it might run slow if the CPU is also running a lot of data through RAM. I'd open system monitor while you're running Lightroom and see if you're hitting a bottleneck.
If you're worried about the photos themselves being blurry, meet with the photographer and have them run through the photos quick. Otherwise, trust the photos are rendered correctly for purchase and pay them for their service.