What does it mean when throughput in the system is constant and only response time changes (linearly)?
Does it mean that I have to have a bottleneck?
If, at increasing load, throughput does not change, this generally means that your system is running at full capacity. If, on the other hand, a load increase brings added throughput (even if it costs some latency), your system is not running at maximum capacity.
A good example are disk IOPS: at low load (ie: QD1), they often are much lower than what you can expect at high load (ie: QD16).