Ah, I remember! wRoNg sCaLe! If that's starship then I'm santa claus Yout TWR is over the roof, causing lots of drag and slaughtering your performance. Those fins are utterly gigantic, the rocket is like 2x smaller than it should be, your RCS spam adds lot's of drag, adding up with the high twr, and those struts man... Your performance = x_x
Summarizing, it's a great rocket, provided that you fix this and some more stuff.
One more question...
Why do you have a payload bay that doesn't open?