Long stroke engines make good torque at low rpm - but are limited in engine speed because the long stroke makes for high piston speeds (up/down).

Shorter stroke engines don't make as much torque at low rpm because the shorter stroke of the crankshaft doesn't provide as much leverage for the piston rod to push on - but the shorter stroke reduces the speed of the piston at high rpm's.

As an example the LS650 engine has a listed HP of 31 HP at 5,200 rpm, and a torque value of 50 nm at 3,400 rpm.  The LS650 has a 94mm bore and a 94mm stroke.

The second generation Suzuki DR650 is a very similar engine - but it has a  shorter 82mm stroke and a larger 100mm bore.  The engine is in a higher state of tune and has a better flowing head and a more aggressive cam grind - and it makes more HP and torgue....but it occurs at a higher rpm.  The HP number is 46 HP at 6,800 rpm and the torque is 56.6 nm at 5,000 rpm.

In comparison the Ducati Panigale V-Twin 1,198 cc engine makes 195 HP at 10,750 rpm and 133 nm of torque at 9,000 rpm - using a 112mm bore and a short 60.8 mm stroke.  (Cut in half this would be a 599 cc engine that makes 97.5 HP and 83 nm of torque).

The long stroke of the LS650 allows it to develop the maximum torque at 1,600 rpm less than the DR650.....and 5,600 rpm less than the Ducati.  Long strokes are great in tractors, trucks, boats...things you want to have lots of pulling power.....more than you want higher rpm or higher HP to weight ratios.

For the LS650 the piston speeds are 3,330 fpm for the 5,400 rpm HP value, and 2,097 fpm for the 5,000 rpm torque value, and 4,009 fpm a the 6,500 rpm redline.  
For the DR650 the piston speeds are 3,658 fpm for the 6,800 rpm HP value, and 2,690 fpm for the 5,400 rpm torque value.  
For the Ducati the piston speeds are 4,288 fpm for the 10,750 rpm HP value, and 3,590 fpm for the 9,000 rpm torque value. (The Ducati has  lightened engine internals, including titanium connecting rods).

The bottom line is that the LS650 does not have exotic materials, and it is made to be a durable low speed engine.  The 6,500 rpm does not have a piston speed that is unusually high at 6,500 rpm - but it is a rather large and heavy piston and it most likely doesn't like those kind of speeds for too long.  Folks who do run at the 70-80 mph speeds for any duration report an increase in oil consumption - not sure if the oil is going past the rings or out the breather.
 
If you are bumping the performance of the engine up - you should build the engine to make more torque below the 6,500 rpm limit - higher rpm values really aren't realistic on this engine.