Where should i look to improve slow I/O performance measured with dd?
My experience with dd is that as well as the file size, the block size makes a huge difference. Try playing with the bs parameter (try 128 - 10240) and see what sort of a difference it makes.

Also - a sustained 28MB/sec write speed is not all that bad for a 5-7 year-old computer. (Although if you multiply by 8 to get the speed in bits per second, you will see that it's 224mbps - around 15% of a SATA-I bus.)

Things to consider include:


Is the HDD a 5400rpm or 7200rpm model? For sustained larger file writes (as opposed to many small files), it makes a difference.
Not all HDDs are created equal, with some models being faster than others. Find out what model yours is and search a bit to find out how they usually fare.
Have you considered upgrading to an SSD? This will make a huge difference, even if the interface on the computer is only SATA-I (quite likely), as a decent SSD should be able to saturate the SATA bus.