Dear Lrtimelapse users,
I am looking to buy a newer pc,
Currently i have a custom windows desktop:
i7 5820k 
32gb ddr4
R9 280x
And looking to buy a iMac 27inch 2017 with i7 and a radeon pro 575 or 580.
Is a 7700k faster in rendering/calculating lrtimelapse than a 5820k ?
I render 24Mp raw pictures and later maybe upto 36-42Mp.
Also would it be better to get a custom pc with nvidia gpu for warp stabilizer?

Thanks in advance.

You'll get more performance for your money, if you buy a custom PC instead of a mac. There are lots of benchmark sites and custom PC assemblers that you could check online for some comparisons and custom configurations.
I'd focus on one or two fast M2 SSDs, enough RAM (32GB at least), a good graphics processor (but not necessarily the fastest one, since the impact is typically not so high), and a fast processor with good multi-core performance.
Check out the new AMD Ryzen processors too, they offer a lot of performance for the money.