However dense the data is on the platter, there is still the latency problem. Latency is the time it takes before a drive is ready to start reading or writing actual bits. For small data reads and writes, the latency takes more time than the actual read or write.
For any read or write, the platter has to rotate under the head to locate the start of the data block. This takes an average of half of the rotational period of the drive (under ideal conditions. Head transit time and logical processing time may make it take more than one rotation by the time the head is ready to read - but the relative measure is still applicable)
4200 RPM drive rotational latency average is .12 microseconds
5400 RPM drive rotational latency average is .09 microseconds
That's a 25% reduction in latency; this affects almost every operation that the drive does. The exception is streaming a large block of contiguous data, uninterrupted by other requests, in that case latency is minimized.