In my understanding Colormix function is working as same as earlier . Now , In your case its all about data . You can check in previous version of Consumer Sales App which comes by default , On first sheet the three bar charts have same function and same GUI.
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ColorMix1() function expects a number between 0 and 1 as first argument and I can't see a reason why the performance of color mapping should have degraded.
Reading your post, I think your expression is more complicated (i.e. the matching of your value you want to map against ranges ">1", "<0" and "inbetween 0 and 1" may include some complex repeated calculations) and maybe the performance of your value calculation has degraded.
Could you at least post your complete, full expression you are using as color expression?