Aside from overclocking, the k SKU usually gets a slightly higher base and boost clock so it is a little bit faster out of the box. In the case of the 10900 vs. 10900k, the k SKU gets a considerably higher base clock of 3.7GHz, plain 10900 is 2.8GHz. Single core boost clock is also 100MHz higher on 10900k 5.3GHz vs. 5.2GHz.
Now things like CPU power limits and Turbo durations can often be overridden in the BIOS which could close the gap between the CPUs, however if you do that, you need an aftermarket cooler for a plain 10900, the Intel stock cooler frankly isn't even adequate for heavy load use under the stock power and Turbo duration limits on Intel's high core count chips.