Usually a fin and tube (stock style) IC heat soaks (gets hot) quick, but also sheds heat (gets cool) quick by design. Bar and plate IC (most of the aftermarket ones for us, minus Mountune) tend to take longer to heat soak, but sheds heat slower.
You really want a good balance, because you can have a solid core that takes forever to heat soak (Garret cores) but if it gets hot, it also takes forever to shed the heat back down.
The stock IC is the opposite, it takes very little to heat soak, but once it gets hot, it sheds heat relatively quickly to a point, but its still not enough obviously. Stock IC sucks, everyone should buy an aftermarket IC.