It's also generically an unwise assumption because you have to know how fast the compression is compared to disk I/O. If, like almost all servers these days, you have more CPU than I/O capacity the compression overhead will often be buried in the I/O latency and if the data compresses well it it's easy to have it be faster because the I/O savings is greater than the CPU.

Since LZ4 was designed to be very fast, that seems like a reasonable bar to hit — I see single-core performance on old desktops in the 1.8+GB/s range.