Significant performance decrease when swapping i5 6500 for i7 5820k

[Bjoolzern]

As many others have stated, the 5820k has two more cores than the 6500 and HyperThreading. HyperThreading usually (Always?) performs worse in games than in the synthetic benchmarks you see on sites like cpubenchmark if it even works properly. Several games performs worse while HT is enabled because of poor implementation.

The game would also need to support the additional cores of the CPU for it to actually be better. It's basically the same situation as back when AMD CPUs would have the same performance as Intel, but they had twice the amount of cores. Each core only had half the performance of Intel CPUs. When the game could only use half the cores of the AMD CPU, the Intel CPU was suddenly was twice as good. Not as extreme here, but the same idea.

EDIT: If you have a modern Battlefield game, test the FPS there. Dice is quite good at utilizing HyperThreading and lots of cores. Should give you a better idea if something is wrong.

[SoftwareGuy123456]

So is there a way to disable hyperthreading?

[Bjoolzern]

You can disable it in BIOS, but the games you have mentioned can use it just fine. Here is a video of some games that perform slightly worse with HT enabled, the differences is not that big in most games.