It seems to me, this would be easier on the saw like it is on the table saw to cut half a board, then, the other half? thanks for input, t
It depends on the size and type of the saw.
In the picture above the 075 (i.e. a powerful machine) is using a short bar and will have no problem making those cuts.
However, trying to do this with a small, plastic saw may end up killing it.
The speed with which the cut is made by the 075 in that photo above is less determined by the power of the saw and more by the chain setup and chain speed.
That saw could easily use a bar double that length and cut the wood in one pass in not much longer a time than it takes to make a half pass with the shorter bar.
The other MAJOR disadvantage is that with the bar buried in the wood Auxiliary bar oiling at the nose cannot be provided.
If a powerful saw with a short bar is used a faster cutting speed can be achieved (up to a point) by significantly lowering the rakers which then places a greater load on the engine - which is what it sounds like you are trying to avoid? so you are then back to swear one. On a really short bar the rakers cannot be dropped far enough before sawdust clearance becomes and issue and the other thing that lower rakers will contribute to is a poorer finish.
With half cuts the hardest cut will be the first one when the bar is buried up to the nose and the maximum load will be on the engine.
If you wanted to cut a 40" log using a 20" bar the smallest saw I would do that with is about a 70cc quality saw using lopro chain.
I have a mill similar to the one show above and I much prefer using a regular alaskan mill which supports the bar at both ends.
On a CSM where the bar is held only at one end if the chain or bar are not setup accurately the bar can wander in the cut.
