Other than Core i9, the rest of the 9th-gen lineup offers just incremental improvements.

Now that Intel’s latest 9th-gen Core mobile chips are on their way, it’s time to figure out whether it’ll be worth it to pay top dollar for the new chip; buy or keep a laptop with an 8th-gen CPU (plenty of current models remain available), or upgrade from your 6th-gen or 7th-gen model.

The issues differ from generation to generation. In many cases, we’ve said you can wait a few years before upgrading. The 8th-generation jump was an exception: It represented one of the biggest laptop CPU improvements in a long time, worthy of upgrading even from a 7th-gen chip.
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https://www.pcworld.com/article/3390994/intel-8th-gen-vs-9th-gen-core-cpu-which-should-you-buy.html