I don't think that's true. C is often slower than C++ because of how inlining works, plus some domains (e.g. compilers) it's best to just use a GC language from the get-go.
Hang on, you’re cutting a few corners there! It’s easy to use inline functions in C too.
Are you thinking of C++ templates, and the fact that e.g. std::sort is faster than qsort because it directly calls an inlined comparison function rather than a function pointer?
That’s true, although it’s possible to achieve similar performance levels in C via hand-rolled data structures or macro hackery. I’ll grant you that the efficient C++ code is more idiomatic and likely safer. On the other hand, idiomatic C code is likely smaller when compiled, which can be important for performance too.
I don’t believe “C is often slower than C++” is true in general.
And when it's not irrelevant, it's almost 100% certain that std::sort is not the right thing to use. Where it matters, it's probably possible to examine the context a little more closely and come up with a custom sort that runs in O(n) or at least faster than std::sort.
std::sort( begin( c ), end( c ), []( const auto& lhs, const auto& rhs ) { /* some comparison impl for the value_type */ } );