Sequential is the speed at reading/writing big files. So if you were copying a bunch of 1GB movie files (which weren't fragmented), the sequential speed is what you'd expect. For a modern 7200 RPM drive it ranges from about 80-160 MB/s today. 4k is the speed at reading/writing really small files (4 kB in size).
Similarly, you may ask, is a 5400rpm hard drive slow?
Spinning hard drives are cheaper and able to hold more data, so cheap computers typically boast 1Tb or 2Tb of storage space. It's not obvious that those big drives are cheap and slow. Solid state drives are typically 256Gb or 512Gb. Spinning hard drives spin at two different speeds, 7200 RPM and 5400 RPM.
Is a 5400 RPM hard drive good?
But today's most common RPM rates, in both laptop and desktop PCs, are between 5400 and 7200 RPM. Given two identically designed hard drives with the same areal densities, a 7200 RPM drive will deliver data about 33% faster than the 5400 RPM drive.