c++ – Need assistance with 2D participant controller

I am making an attempt to create a 2D side-scrolling participant controller like in Terraria, however I can not get the delta time proper. If I set my laptop computer’s mode to battery saver, my character jitters (from dt jitters), however on efficiency, all the pieces is easy. It additionally seems that the decrease the framerate is, the sooner the participant strikes.

   auto dt = GetFrameTime();
   auto dir = IsKeyDown(KEY_D) - IsKeyDown(KEY_A);

   if (not onGround) {
      vel.y = min(maxGravity * dt, vel.y + gravity * dt);
   } else {
      vel.y = min(maxGravity * dt, gravity * dt);
   }

   if (dir != 0) {
      auto speedX = (onGround ? pace : pace * .6f);
      vel.x = lerp(vel.x, dir * speedX * dt, acceleration);
   } else {
      vel.x = lerp(vel.x, 0.f, deceleration);
   }

   if (IsKeyDown(KEY_SPACE) and canHoldJump) {
      vel.y = jumpSpeed * dt;

      holdJumpTimer += dt;
      if (holdJumpTimer >= jumpHoldTime) {
         canHoldJump = false;
      }
   }

   if (onGround) {
      canHoldJump = true;
      holdJumpTimer = 0.f;
   } else if (not IsKeyDown(KEY_SPACE)) {
      canHoldJump = false;
   }

   pos.y = lerp(pos.y, pos.y + vel.y, smoothing);
   // Deal with Y collision and floor examine
   pos.x = lerp(pos.x, pos.x + vel.x, smoothing);
   // Deal with X collision

Listed below are my constants if it helps:

constexpr float pace = 80.f;
constexpr float jumpSpeed = -100.f;
constexpr float gravity = 10.f;
constexpr float maxGravity = 200.f;
constexpr float acceleration = .083f;
constexpr float deceleration = .167f;
constexpr float smoothing = .333f;
constexpr float jumpHoldTime = .4f;
```