Did you delete fxo and metashaders2 folders? What in general helped me, as the update to 2.5.6 caused a big performance drop on my pc, was to start with a new DCS.openbeta user folder. To test if that has any effect, you could rename the folder, so you can easily revert back, if it didn't help.
His CPU has a Max. Turbo-freq of 4.10 GHz.