(C++) is member function slower than free function?

i was solving algorithm problems and wondering why my code is about twice slower than others even the logic was same.
Then i figured out using lambda or free function is much faster than member function.
i googled about it but most of the people says there are no difference.

i checked excution time on my computer, the results are below:
its a simple fibonacci function within/without class. Member function is about 0.3sec slower than free function.

class bma{
public:
    int dfs(int a){
        if(a <= 1) return 1;
        return dfs(a-1)+dfs(a-2);
    }
};

int dfs(int a){
    if(a <= 1) return 1;
    return dfs(a-1)+dfs(a-2);
}

int32_t main(){
    bma bm;
    time_t start,end;
    double result;

    for(int T=1; T<= 5; T++){
        start = clock();
        bm.dfs(42);
        end = clock();
        result = (double)(end-start);
        cout << T << "th class func exec time:" <<  result/CLOCKS_PER_SEC << endl;
    }

    for(int T=1; T<= 5; T++){
        start = clock();
        dfs(42);
        end = clock();
        result = (double)(end-start);
        cout << T << "th norm func exec time:" <<  result/CLOCKS_PER_SEC << endl;
    }
}

is there something i don't know about C++ or process of calling a function?