shell_test.erl

LoadQueue = fun(N) ->
    lists:foreach(fun(_) ->
        self() ! foo
    end, lists:seq(1, N))
end.

RunTest = fun() ->
    lists:foreach(fun(_) ->
        Ref = termsend:ref(),
        receive Ref -> ok end
    end, lists:seq(1, 10000))
end.

RunTest(). % This is ~instant
LoadQueue(10000).
RunTest(). % This is ~30-60s

termsend.c

#include "erl_nif.h"

static ERL_NIF_TERM
repeat(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
{
    ErlNifEnv* msg_env = enif_alloc_env();
    ERL_NIF_TERM ref = enif_make_ref(env);
    ERL_NIF_TERM msg = enif_make_copy(msg_env, ref);
    ErlNifPid pid;

    enif_self(env, &pid);
    enif_send(env, &pid, msg_env, msg);
    enif_free_env(msg_env);

    return ref;
}

static ErlNifFunc nif_funcs[] = {
    {"ref", 0, repeat}
};

ERL_NIF_INIT(termsend, nif_funcs, NULL, NULL, NULL, NULL);

I've also tried creating the Ref in Erlang and passing it as an argument to the NIF but it didn't change the behavior.