Conclusion

Default

When comparing these two processors you need to keep in mind that we're basically testing to different platforms. As you can see in our test setup descriptions, we're using an Z77 chipset based motherboard with DDR3 memory in the case of the i5-2500K and a Z170 board with DDR4 regarding the i5-6600K. Apart from that it wasn't possible to drive our i5-2500K at higher memory frequencies than DDR3-1866, whereas the i5-6600K can easily do DDR4-3000.
First of all we’re going to have a closer look at performance differences with standard clock speeds. From our overall performance rating we see that the Core i5-6600K is on average 41% quicker than the Core i5-2500K. Searching for benchmarks with best possible scaling on the i5-6600K we find XTU benefitting massively and running more than 66% faster on this CPU than on the i5-2500K. Having a look at a more real world oriented benchmark like Frybench rendering benchmark, we see that the i5-6600K performs 43% faster than the i5-2500K. On the other hand it's interesting to see that with Blackhole's Single Threaded benchmark the i5-6600K pulls away by a whopping 46%.
We also had a closer look at power consumption and in idle we noticed that our test setup with Core i5-6600K was burning 35% less power than our test platform with Core i5-2500K. In case of load power consumption the difference was 1% in the same direction.

4.2 GHz

Overclocking both processors to 4.2 GHz basically makes the Core i5-6600K perform 8% faster and the Core i5-2500K benefits by almost 27%. Apart from that it interesting to see that the performance difference between the two CPUs at the same frequency is almost 28% on average, which is quite a bit less than what we measured running both platforms at stock clocks.
A closer look at power consumption reveals that overclocking these CPUs makes for quite a difference. With the Core i5-6600K we see system power consumption rise by 7% in idle and 4% under load, when comparing with the default power consumption values. In the case of the Core i5-2500K system power consumption goes up by 16%, when the CPU is under full load. In idle the difference 14%.

i5-6600K @ 4.2 GHz DDR4-3000 / i5-2500K @ 4.2 GHz DDR3-1866

The default clock speeds of DDR4 in the case of the i5-6600K is 2133MHz and therefore running the memory at 3000MHz is quite a boost. Concerning the Core i5-2500K we were able to increase memory clock speeds from DDR3-1333 to DDR3-1866. Unfortunately more wasn't possible with our particular i5-2500K due to a weak IMC (integrated memory controller). Overclocking the system with the Core i5-6600K inside we notice that the performance goes up by 13%. In the case of the Core i5-2500K the difference is 26% on average. Comparing the performance of the two systems we see that the setup with Core i5-6600K is almost 28% faster.
Looking at idle power consumption, the system with Intel Core i5-6600K turns almost 15% more energy into heat, whereas with the i5-2500K consumption goes up by 3%. Under full load the differences are almost 16% in the case of the i5-6600K and 14% for the i5-2500K.

Recommendation

If you still own an i5-2500K we can definitely tell you that you will feel a difference switching to a new Z170 based platform with an Intel Core i5-6600K CPU. An average performance boost of 41% is something you'll be able to notice especially if you do some photo and video editing. Neverthelss it's interesting to see how well the i5-2500K still stacks up against the much newer i5-6600K. Considering that there are five years in between the release dates of these to platforms the overall performance difference is not that big. Especially if you keep in mind, that when both CPUs run at the same frequency the i5-6600K is "only" 28% faster, which basically translate into the efficiency increase per clock cycle regarding the Core i5-6600K.

Page 1 - Introduction	Page 8 - Frybench
Page 2 - 3DMark Physics	Page 9 - HWBot Prime
Page 3 - XTU	Page 10 - UCBench
Page 4 - Cinebench	Page 11 - Performance Rating
Page 5 - SiSoft Sandra	Page 12 - Power Consumption
Page 6 - Blackhole	Page 13 - Conclusion
Page 7 - WinRAR / 7-Zip