DDR4 memory at 2133 MHz has 25.6 GB/sec of bandwidth available, and that's peasant tier memory like I run.
The fastest USB spec available today, 3.2, has 2.5 GB/sec of bandwidth available. To approach the bandwidth available to a memory interface, you'd be looking at Infiniband connections, which in terms of cost are priced at "if you have to ask, you can't afford it and probably don't need it."
Honestly, if a computer is so cheap and slow that the memory is an issue, and it can't be upgraded, it's time to chuck it and buy something else. Or give Lubuntu a try.