That sounds like some other things I've heard when using the RGB curves secondaries. Have you tried the Three-Way secondaries?
And yes, there can be issues with just little ... unusual ... occasional ... oddities, with the "legacy" color correction tools. Which is why some of the experienced colorists have avoided doing too much in PrPro up to now, they couldn't predict how far they could go with too many things.
If you can't predict ... makes it hard to make deadlines & budgets.
And yea ... the 2015.4 Lumetri on many "decent modern" machines is a bit more demanding of hardware but still not too much of a pain for renders, playback & etc. In my testing, vastly better than 2015.2 on my modest rig. Working 1080 with some 4k thrown in.
Neil