Most programs rarely issue all the instructions that a CPU can handle simultaneously, they are stuck waiting on memory or linear dependencies. An extra compile-out-able conditional typically doesn't touch memory and is off the linear dependency path, which makes it virtually free.
So the actual real-world overhead ends up at less than 1%, but in most cases something that is indistinguishable from 0.
If you care that much about 1% you are probably already writing the most performance critical parts in Assembly anyway.
I call this hotspot fallacy and it is a common one. This assumes there is relatively small performance critical parts that can be rewritten in assembly. Yes, sometimes there is a hotspot, but by no means always. A lot of people caring about 1% is running gigabytes binary on datacenter scale computer without hotspots.
I'd be curious to know how much production code today that's written in C is that performance critical, i.e. depends on all those bonkers exploits of UB for optimizations. The Linux kernel seems to do fine without this.