The cost of waiting is real too. Rents and prices both tend to move while you save, so the extra years spent reaching 20 percent are not free, and there is no guarantee the market moves in your favour during that time.
The cost of buying early is the CMHC premium itself, which gets added to the mortgage and paid off over the loan's life, plus the PST on that premium due upfront. For many buyers that upfront cost is smaller than the years of rent saved by not waiting.
Joel Dyck can run both scenarios against a real property so the decision is based on numbers, not a rule of thumb. Get your hand-reviewed valuation from Joel Dyck.