Fix extreme frame drop when running into certain corners (Fixes #2023)

apps/openmw/mwworld/physicssystem.cpp

@@ -350,6 +350,8 @@ namespace MWWorld
                 velocity *= 1.f-(fStromWalkMult * (angle.valueDegrees()/180.f));
             }
 
+            Ogre::Vector3 origVelocity = velocity;
+
             Ogre::Vector3 newPosition = position;
             /*
              * A loop to find newPosition using tracer, if successful different from the starting position.
@@ -427,10 +429,18 @@ namespace MWWorld
                 else
                 {
                     // Can't move this way, try to find another spot along the plane
-                    Ogre::Real movelen = velocity.normalise();
+                    Ogre::Vector3 direction = velocity;
+                    Ogre::Real movelen = direction.normalise();
                     Ogre::Vector3 reflectdir = velocity.reflect(tracer.mPlaneNormal);
                     reflectdir.normalise();
-                    velocity = slide(reflectdir, tracer.mPlaneNormal)*movelen;
+
+                    Ogre::Vector3 newVelocity = slide(reflectdir, tracer.mPlaneNormal)*movelen;
+                    if ((newVelocity-velocity).squaredLength() < 0.01)
+                        break;
+                    if (velocity.dotProduct(origVelocity) <= 0.f)
+                        break;
+
+                    velocity = newVelocity;
 
                     // Do not allow sliding upward if there is gravity. Stepping will have taken
                     // care of that.