If you don't have a lot of computer knowledge interpreting the PerformanceTest results can be confusing. There are however a few simple concepts that can help you.
PerformanceTest executes a collection of different tests on your computer to test different aspects of it's performance. There is a suite of tests for the CPU, Disk, Memory, 3D graphics and 2D graphics. For each suite there is a "Mark" value. For example the CPUmark. These mark values are then combined into a single overall score called the PassMark rating.
The CPUmark value is a measure of the CPU's performance. The PassMark rating is a measure of the entire system's performance. If you want to understand how all the individual scores are combined into Mark values you can find the PerformanceTest formula documented here.
You can find a chart of all the CPUMark values on the CPUbenchmark.net web site. A the time of writing 20,000+ was a high CPUMark while 8000 was more typical for a newish machine.
The Mark values are good for a quick assessment of the hardware's performance. However people use computers in different ways with different software. While an attempt was made by us, the developer, to write benchmark code that resembled real life code used in real applications, it is impossible for any benchmark software to exactly reproduce any particular individual's usage patterns. Some computers are used for gaming, some for web servers, some for office tasks. So you need to apply some common sense when interpreting the results. For example the 3DMark value isn't particularly relevant to an office worker.
For more details of the benchmark tests performed, see the Help file included with the PerformanceTest software and the CPU test description page, Graphics test description page, Disk test description page and RAM test description page
If the Mark value is doubled, does this mean double the performance?
The vague wishy washy answer is: Yes it does, some of the time, at least for a limited set of circumstances.
Taking the CPUMark as an example. The CPUMark score is mostly made up of benchmark algorithms that A) execute almost exclusively on the CPU and B) Fully uses the all the CPUs cores that are available. There isn't any point, for example, having a CPU benchmark whose result is linked to the speed of the hard disk. In more technical terms the CPU benchmark is CPU bound. However many real world applications are not CPU bound. They spend some of their time waiting for the hard drive to read a file, some of their time receiving data from the Internet, some of their time updating the display, etc. Also many real world applications are not very well "threaded" and only run on one CPU core. So for these applications you won't see double the performance from a doubling in the CPUMark.
For poorly threaded applications that run on a single CPU core, it makes more sense to look at the single threaded benchmark chart. as this will give a much more realistic indication of performance compared to the main CPU benchmark charts.
Also the CPU test has a small dependence on the RAM speed, so at least for the faster CPUs, better RAM can make the CPU look slightly faster. Likewise the 3D, 2D & RAM tests have some dependencies on the CPU speed. So upgrading to a new video card which in theory is double the power, might not give the desired results if the performance is being bottlenecked by a slow CPU.