I don't think there's much of an argument here. Maybe there's a difference between how a computer scientist may see things and how a normal end-user may see things, but in real life, multitasking many apps can very often reduce the responsiveness (which many people would describe as "slow down") a device. This is common knowledge on Windows, Macs, and Maemo.

Personally, I like the fact that I can choose to multitask, unlike on iPhone, but I don't kid myself that it has no impact on the responsiveness of the device.