With Canada's comparatively generous social entitlements (compared to the US) one could reasonably wonder if the lag between a newcomer arriving and making a net economic contribution is on balance too large.  

The government ostensibly assumes that certain immigration classes take a generation to reach that point of net contribution. But in the meantime, they are a net load to the system given those generous entitlements.

So, an increase in the immigration rate (beyond 300k/yr) without changes to the immigration class allocation or to entitlements, could result in a peak economic load from the combination of baby boomer retirements and new immigrants.  The fiscal consequences (more debt and tax) of this peak could well undermine the promised benefits in the subsequent generation of integrated and contributing immigrants.  

Or have we already passed that inflection point of diminishing returns at a 300k/yr rate?   

Am I missing something here?