Sometimes it’s because they use cheap Shingled Magnetic Recording (SMR) drives in the external enclosures. These drives are slower and less reliable than the Conventional Magnetic Recording (CMR) drives. Some of the cheaper internal drives are also SMR these days. An SMR drive is OK if you just want to write data to it and leave it there, and you’re not too worried about the transfer speeds. But if you’re doing a lot of intensive activity with many writes and rewrites, it’ll slow you down.