I noticed some time ago when I was hiking and backpacking weekly summer, and winter. Winter pack loads are heavier due to carrying extra clothes and gear in addition to already wearing heavier clothes, insulated boots and wearing snowshoes. I noticed then that my friends would begin to warm up in minutes and begin unzipping or removing parkas where I was still warming up 20 minutes later. I've noticed the same thing on the bike. It takes about 20 minutes to feel 'warmed up'.