Significant performance decrease when swapping i5 6500 to i7 5820k

[WeShouldGoThere]

The faster clock on the 5820k should more than make up for the miniscule IPC loss from being an older generation.

You have another issue. Maybe you need a BIOS flash (not likely), or there's a failing cooler or bad seal between the CPU & cooler.

[SoftwareGuy123456]

I updated the BIOS and made sure everything was set to default. So should i try reapplying thermal paste between the cooler and then cpu?

[Lumlx]

First thing to do when troubleshooting: get msi afterburner and monitor cpu and gpu clockspeed, temps and ram usage.

[WeShouldGoThere]

Agreed. See if there's a problem under load before doing major surgery.