Ray-Box (AABB) is slower than without

at the moment im trying my best to write my own raytracer (and its quite fun actually). So the last days I tried implementing a bounding box algorithm to it. But im getting a much slower framerate with the bounding boxes turned on :(. I think it has something to do with checking the box with every ray but I dont know how I could change that. Here is my code:

the Intersection algorythm

bool Intersect(Ray r, float3 lb, float3 rt) {     float3 dir_inv = 1 / r.direction;          double t1 = (lb[0] - r.origin[0]) * dir_inv[0];     double t2 = (rt[0] - r.origin[0]) * dir_inv[0];      double tmin = min(t1, t2);     double tmax = max(t1, t2);      for (int i = 1; i < 3; ++i)     {         t1 = (lb[i] - r.origin[i]) * dir_inv[i];         t2 = (rt[i] - r.origin[i]) * dir_inv[i];          tmin = max(tmin, min(t1, t2));         tmax = min(tmax, max(t1, t2));     }      return tmax > max(tmin, 0.0); } 

My trace function:

RayHit Trace(Ray ray) {     RayHit bestHit = CreateRayHit();     uint count, stride, i;      // Trace ground plane     IntersectGroundPlane(ray, bestHit);      // Trace spheres     _Spheres.GetDimensions(count, stride);     for (i = 0; i < count; i++)     {         if (Intersect(ray, _Spheres[i].position - (_Spheres[i].radius), _Spheres[i].position + (_Spheres[i].radius)))             IntersectSphere(ray, bestHit, _Spheres[i]);     }          // Trace mesh objects     _MeshObjects.GetDimensions(count, stride);     for (i = 0; i < count; i++)     {         //if (Intersect(ray, float3(0.0f, 0.0f, 0.0f), float3(10.0f, 10.0f, 10.0f)))             //IntersectMeshObject(ray, bestHit, _MeshObjects[i]);     }      return bestHit; } 

thanks in advance