Although std::unordered_map is definitely not nearly as fast as a hash map can be in c++, that's probably not the main problem here. I would bet on iostreams an

Although std::unordered_map is definitely not nearly as fast as a hash map can be in c++, that's probably not the main problem here. I would bet on iostreams and/or tolower being bigger problems. Note that tolower is called for every /character/. I don't know everything it does, but I know it's doing some semi-esoteric locale stuff, and in general it's doing a lot more than a naive ascii implementation would. I seem to recall it's even doing a dynamic_cast in there somewhere. For each character of input. I was curious, so I ran callgrind on it. Here are the top 10 offenders (by Ir, 'instructions retired'). For clarity I have elided many details. Total Ir is 5,647,099,795, so the stuff below accounts for greater than 50% of all Ir. 1,079,594,033 std::istream& operator>>(std::istream&, std::string&) 553,657,533 std::istream::sentry::sentry(std::istream&, bool) 550,159,646 __cxxabiv1::__vmi_class_type_info::__do_dyncast(...) 426,992,280 __dynamic_cast 413,240,330 std::_Hash_bytes(void const*, unsigned long, unsigned long) 383,294,340 tolower 230,374,111 std::string::_M_append(char const*, unsigned long) 223,653,549 /usr/include/c++/9/bits/stl_algo.h:main 172,438,098 __strcmp_avx2 164,226,680 std::ctype const& std::use_facet(std::locale const&)