High number of transitive includes causing bad compile times

I really like the implementation quality of this library. However, consider the following test program:

#ifdef STD_UNORDERED
#include <unordered_map>
#endif
#ifdef BST_UNORDERED_FWD
#include <boost/unordered/unordered_flat_map_fwd.hpp>
#endif
#ifdef BST_UNORDERED
#include <boost/unordered/unordered_flat_map.hpp>
#endif

int main()
{
}

and observe the compile time with different preprocessor configurations (after prewarming the filesystem cache):

λ  time g++-12 -std=c++20 -O2 -I/home/me/ldevel/vcpkg/installed/x64-linux/include compile-test.cpp -o compile-test
g++-12 -std=c++20 […] 0.01s user 0.02s system 99% cpu 0.033 total
λ  time g++-12 -std=c++20 -I/home/me/ldevel/vcpkg/installed/x64-linux/include -O2 -DSTD_UNORDERED compile-test.cpp -o compile-test
g++-12 -std=c++20 […] 0.15s user 0.02s system 99% cpu 0.170 total
λ  time g++-12 -std=c++20 -O2 -DBST_UNORDERED_FWD -I/home/me/ldevel/vcpkg/installed/x64-linux/include compile-test.cpp -o compile-test
g++-12 -std=c++20 […] 0.49s user 0.02s system 99% cpu 0.517 total
λ  time g++-12 -std=c++20 -O2 -DBST_UNORDERED -I/home/me/ldevel/vcpkg/installed/x64-linux/include compile-test.cpp -o compile-test
g++-12 -std=c++20 […] 0.75s user 0.03s system 99% cpu 0.782 total

flags	time
	`0.033s`
`-DSTD_UNORDERED`	`0.170s`
`-DBST_UNORDERED_FWD`	`0.517s`
`-DBST_UNORDERED`	`0.782s`

System information:

g++-12 (Ubuntu 12.1.0-2ubuntu1~22.04) 12.1.0
CPU: Intel(R) Core(TM) i9-9980HK

The issue seems to be that unordered_flat_map always includes boost/container_hash/hash.hpp which in turn includes large parts of the standard library (<string>, <complex> => <sstream>, variant).

It seems somewhat unfortunate to me that the compile time is increased that much, especially since the unordered_flat_map is usually part of a composite data structure and therefore transitively included in many TUs.

Furthermore I usually don't use boost::hash but a custom hash implementation (SpookyHashv2, xxHash or domain specific hashes) and therefore would like to ask whether it'd be possible to provide a variant which either uses std::hash as default Hash parameter or doesn't supply one at all.

boostorg / unordered

High number of transitive includes causing bad compile times #195