Complex sentence: First, it used an existing operating system, CP-V, as a model; though many improvements were made and weaknesses corrected, the existence of a fundamentally correct working model saved much design time and even more discussion and argument.
Split into simpler sentences: First, it used an existing operating system, CP-V, as a model. Though many improvements were made and weaknesses corrected the existence of a fundamentally correct working model saved much design time and even more discussion and argument.