I would leave Registry alone.

It could be safe to play with it if you are able to get inside the head of every programmer whose software is on your machine.

In practice, VERY few Registry entries will actually be slowing your machine down.

The most common reasons for old systems to be "slow" on start up are:

1. Old systems have slow hardware
2. Modern software is often designed and tested on state of the art macines using tools that produce load of bloat. If you showed the programmers how slow it was on your machine, they'd probably giggle.