I think that declining sales performance can mean that sales is declining. It could be high on it's way down, or going from bad to worse. In other words, there's definitely an element of negative growth in sales.
On the other hand, low sales growth is just that -- the sales is generally flat. It might not be a good or bad thing in itself; you will want to check how this has changed over time and vis a vis the industry. If it's low growth but you are 50% better than the industry average, that's sort of good news.
In both cases, if this comes up in a case interview, it's a good idea to clarify with interviewer. Ask him to help you define and quantify this, and do also check how it's changed historically and how it benchmarks to competition.