Very slow SMB on MacOS with 10G network
I would turn on smb3 signing, smb3 encryption and retest. We are seeing the best performance with the latest OS, newest protocols and full encryption. It’s not twice as fast, but it’s measurably faster for a couple of vendors across the board for us. 

Next, get a wire shark dump and look at the SMB commands. Work with networking on MTU and jumbo packets as that can really vault large transfers and be worth the engineering work if you can’t speed up large reads.