Complex sentence: Bitnet Relay gained popularity in the late 1980s when Valdis Kletnieks at Virginia Tech released a Pascal version from scratch that consumed far less CPU time, and in the early 1990s when Smart Relay appeared, which allowed more impressive handling of message delivery.
Split into simpler sentences: Bitnet Relay gained popularity in the late 1980s when Valdis Kletnieks at Virginia Tech released a Pascal version from scratch that consumed far less CPU time. And then in the early 1990s when Smart Relay appeared allowing for more impressive handling of message delivery.