The problem is that hard drives are often the slowest part of a computer. While everything else is measured in nanoseconds, HDD access speeds are measured in milliseconds. They are the weak link in the chain as CPU and GPU performance doubles every couple of years.

Although HDDs are slower than SSDs, there is a wide range of performance offered to consumers. Most hard drives in the 2.5 inch mobile HDD market range from 4200 RPM to 7200 RPM, with varying amounts of DRAM cache. The slower drives are typically cheaper and sap less battery life.

However, the difference in power consumption and battery life between 7200 RPM and 5400 RPM drives continues to decline on a yearly basis. Larger versions of 7200 RPM HDDs come out every year, close to or at the same time as their 5400 RPM counterparts.

The problem is that OEMs like Dell, HP, and Lenovo often charge a large premium when moving to faster and higher capacity hard drives. It is often cheaper for consumers to purchase an upgrade and install it themselves, rather than have it upgraded at the factory. Price-sensitive or unknowledgeable consumers may instead stick with what the OEM has selected for them,

Things are about to change. Challenged by the near-instantaneous boot times of SSDs, many OEMs are starting to offer more 7200 RPM HDD options at lower price points. Some are even using 7200 RPM drives by default, especially in desktop replacement and workstation notebooks.

The performance difference can be quite large. 7200 RPM drives have a theoretical 33% performance advantage over their 5400 RPM peers. However, the performance delta can be even greater if the 7200 RPM drive has more cache and higher areal density, which enables the spindle heads to read more data.

Seagate is the leader in sales of 7200 RPM HDDs, with a 44% market share. They expect notebooks and netbooks to transition very quickly from 5400 RPM to 7200 RPM drives, with 50% adoption in 2011. The majority of mobile PCs will use 7200 RPM hard disk drives in 2012, according to their market research.

No doubt new mobile hard drives featuring larger cache sizes, support for 6Gbps SATA, and larger capacities are forthcoming, but when we'll see them still depends on consumer demand. The first mobile chipsets from Intel supporting 32nm Sandy Bridge processors and 6Gbps SATA are expected to hit the market in early 2011.

You might not be able to afford a SSD right now, but a 7200 RPM HDD upgrade might be an option worth considering.