If I assume your spinning disk does roughly 100 MB/s that would indicate you’re getting around 300+ MB/s. That’s not bad, but it’s still low compared to ~500 MB/s you see on even pretty inexpensive consumer grade SSDs. Almost as if there were an old SATA bridge in there (3 Gbps). Highly unlikely. I’m starting to wonder if UASP on Macs is a bigger deal than I thought. OTOH, IIRC a 2015 MBP’s USB3 peak bandwidth is 5 Gbps so maybe you’re just plain bus limited and the only way around that would be a newer MBP that supports 10 Gbps over USB-C (3.1 Gen 2).