X = Time/Task, R = Time/Task
Load:- how much work is incoming? or, how big is the back log?
Utilization:- how much of a system's resources are used?
Scalability:- what is the relationship between utilization and R?
Throughput:- X - how many tasks can be done per unit of time?
Concurrency:- how many tasks can we do at once?
Capacity:- how big can X go without making other things unacceptable?