It is a myth that blended cements sets slower. BIS specification for setting time is same for all these cements. In fact we compare the setting time of concrete / mortar made with these cements.
The rate of hardening of concrete made with PPC/PSC is slightly lower in comparison to concrete made with OPC. This is advantageous as higher rate of hardening will result in early loss of water from concrete which is highly undesirable. In fact concrete made with PSC will have a higher ultimate strength.