I actually have a base M2 Ultra 1TB model and it's very fast - over 6,000 MB/sec, a little faster than the M1 Ultra. But yes, the 512GB models are half as fast (read this forum, there are owners). Still 3,000 MB/sec isn't "slow". I edit off of NVMe drives on Thunderbolt that run at that speed (at best! It's usually closer to 2K). I have zero issues, even with multiple 8K streams.