Need help with 2D player controller

I'm trying to create a 2D side-scrolling player controller like in Terraria, but I can't get the delta time right. If I set my laptop's mode to battery saver, my character jitters (from dt jitters), but on performance, everything is smooth. It also appears that the lower the framerate is, the faster the player moves.

   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 ? speed : speed * .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);
   // Handle Y collision and ground check
   pos.x = lerp(pos.x, pos.x + vel.x, smoothing);
   // Handle X collision

Here are my constants if it helps:

constexpr float speed = 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;
```