Complex sentence: At the time the Cray-2 was being designed, the state of GaAs manufacturing simply wasn't up to the task of supplying a supercomputer, but by the mid- 1980s, things had changed and Cray decided it was the only way forward.
Split into simpler sentences: At the time the Cray-2 was being designed, the state of GaAs manufacturing simply wasn't up to the task of supplying a supercomputer. By the mid- 1980s, things had changed and Cray decided it was the only way forward.