2002 Mitsubishi Diamante slow acceleration

Please help….I just recently encounter this problem. From a complete stop to acceleration is very slow. Let put it this way when I hit the paddle it feel like my car is dragging a boat or something and when I do 70mph on highway the rpm reach to 3 or 4 mark which usually don’t do that. And another problem, I think my car starting to suck A lot of gas even though I just tune-up, replacing gas filter and sparkplug etc... Please Help, appreciate.