Like many misconceptions, the one you hold is based on fact.
The idea of the pre-fetch is to make it possible for Windows and the
applications you use most often to load faster, by 'pre-loading' certain
components. It's a smart idea that works well.
When you change your mind about which applications you use most often, it
takes the pre-fetch a little while to catch on. Meanwhile, it's continuing
to pre-load components of applications that are no longer on your Top 10
list. Maybe they're not even on your computer anymore. This slows your
computer's performance, technically, but you would be hard-pressed to notice
it, given how fast computers work these days.
The pre-fetch folder is constantly being updated, so eventually Windows does
'catch on' and adjusts. If that doesn't happen fast enough for you, you can
manually clear out the pre-fetch folder. But that's like throwing the baby
out with the bath water, because now Windows and the applications you are
using most often will start slower as the pre-fetch is being re-built.
That being said, you would want to clear out the pre-fetch if it's trying
load components of adware or spyware.
So, to sum up: Cleaning out the pre-fetch will not "speed up opening
programs that have slowed opening." If anything, that will make them open
even more slowly. Is there any risk in cleaning out the pre-fetch? No - it's
constantly being re-built.
Rocky