Their virtual warehouses usually take only a few seconds to spin up, but can take longer (especially for large warehouses).
If you keep the warehouse running, there's a fair amount of caching going on behind the scenes that can make your queries run faster on the warm warehouse (how much faster depends on the query)
.
This article shows one example - a cold query ran in 20 seconds, after caching it ran in 1.2 seconds on a warm warehouse:
https://community.snowflake.com/s/article/Caching-in-the-Sno...
So you don't always want to set the idle shutoff time to the minimum.