Fancy new features have been hurting performance since the beginning. Nobody notices it because new, faster hardware makes it a non-issue.

For example, in 1998 I could browse the web perfectly fine on a 100 Mhz Pentium with 32 Mb of RAM. If new features didn't hurt performance then I would still be able to browse the web perfectly fine on a 100 Mhz Pentium today.

box-shadow is just a specific example. In five years even the crappiest smart phone will have enough CPU/GPU power to display box-shadows without any noticeable slow down.

Just go man... Mhmm, warm.