Assuming you're comparing 'equivalent' Skylake and Kaby Lake CPUs, e.g. i5-6200u vs i5-7200u, you basically get a 200-300 MHz clock boost by going with Kaby Lake. Performance difference won't be huge, but you could theoretically get ~10% better CPU performance with Kaby Lake for the CPUs I listed, which is actually not bad considering how much generational CPU improvements have slowed down these days.