Creation time or Build time is the time required to recruit a unit. Creation time is measured in game-ticks. 42-43 (simplified to 42.5) game-ticks are equal to roughly one second in real-time when measured in default game speed.
Creation speed upgrades only affect the production speed and do not "directly" reduce the creation time. Because of this, a 200% production speed upgrade is not four times as effective as a 50% production speed upgrade. This causes the upgrade-efficiency to roughly look like this.
The function displayed in the graph may not be accurate and only serves to demonstrate the falling trend of upgrade effectiveness, the higher the upgrade percentage goes.