It can slow down the speed of the web interface if that's what you're asking. Editor is the way to go.
I mean the ads won't show as much.... I think I'm not actually sure what they mean. But basically, what problems will you face with a new account?
If you are working with the AdWords Editor, it can slow a bit depending on your computer's assets. The quantity of keywords will not affect the frequency your ads are displayed. You need to keep an eye on your daily budget. As you add keywords, you are likely getting more impressions and clicks which are using up your budget. If you are hitting your daily budget often, then your ads will not display as much as before.
That's good news then. I actually have a lot of keywords like maybe 10,000s targetted keywords that I intend to put into my account by today. But I was warned not to do so much so quickly.