Faster way to read/write a std::map or std::unordered_map from/to a file

I am working with some very large std::maps (hundreds of millions of entries) and need to save and load them to and from a file. The way I am currently doing this is by iterating through the map and reading/writing each key and value pair one at a time:

std::map<unsigned long long int, char> map;

void save(){
    std::map<unsigned long long int, char>::iterator iter;
    FILE *f = fopen("map", "wb");
    for(iter=map.begin(); iter!=map.end(); iter++){
        fwrite(&(iter->first), 8, 1, f);
        fwrite(&(iter->second), 1, 1, f);
    }
    fclose(f);
}

void load(){
    FILE *f = fopen("map", "rb");
    unsigned long long int key;
    char val;
    while(fread(&key, 8, 1, f)){
        fread(&val, 1, 1, f);
        map[key] = val;
    }
    fclose(f);
}

But with around 624 million entries, reading the map from a file took 9 minutes. Writing to a file was faster but still took several minutes. Is there a faster way to do this that will also work with std::unordered_map?