This is correct, the 256gb is substantially slower as uses fewer physical memory chips - less ability to read/write in parallel. The 512gb or larger models have substantially higher read/write rates, and typically performs 50-100 percent faster in benchmarks than the 256.

Was a primary factor in me buying a 512gb M4 Mac Mini, even though I planned to use large external SSD - I wanted faster spec boot volume.

Yeah, its the 256gb version.