That is a benchmark, and you should expect higher than average temperatures, but 90C for that cooler is a bit much.
90C isn't terrible for an i5-7600k at like 5Ghz or something, quite high for stock settings. Though many motherboards will apply way too much core voltage, particularly if you are using any of the preset overclocking profiles. Those are set up for maximum compatibility. Anything above 1.35 core voltage is a little high for daily use. Stock would be around 1.2-1.25 volts for 4.2Ghz. 4.5Ghz should be achievable at around 1.3 volts, typically.
You probably don't have the cooler properly mounted. I thought Corsair would have solved this by now. I know on older models they started shipping washers. The cause is overly thin motherboards.
Essentially if you have the cooler down as tight as it will go on its mount, it may still not be in full contact with the CPU. The common solution is to add small washers between the backplate and the motherboard. This leaves less room for the CPU and board thickness to fit between the two clamping surfaces, and increases the clamping pressure. You can sort of test this by pressing on the cooler during operation. If it gets cooler, then it is not in full contact.
Don't go overboard on tightening though. If you have some fresh thermal compound handy, first take the cooler off and see how well the thermal paste spread out. If it looks uneven that is another sign. You can just try a re-install and see if it gets better.
Some Kabylake chips did come from the factory pretty terrible, so de-lidding and replacing the thermal compound underneath the CPUs heatspreader is another potential solution. I did it and it dropped 6 or 7 degrees off one of my cores that was running much hotter than the others.
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