What is getting slower? Playback in preview?
How are you adding clips? To the timeline on the same track? Different tracks?
The only thing that should cause it to slow down is adding clips on different tracks (if you have three clips under the cursor, it has to work on three timeas as many clips at once as if you have one) and effects/trasitions/changes to opacity/pans/crops/track motion)
BUT if you simply have one video track and use straight cuts with no effects adding more clips to the end will not slow vegas down.
On the other hand, if you don't have enough hard drive space or it's not defragmented you may get problems. Anything else accessing your files at the same time might cause problems (anti-virus). And I didn't think such a thing as a Celeron Core 2 Duo existed.