Assuming the table is 200 pages or more (for trivial table sizes, answers are less noticeable):
- Yes. After initial INSERT, if pages become fragmented, those pages will, by definition, be less full, and thus have some empty space.
- For a single, keyed lookup, no. Total pages and even fragmentation do not affect those. For any read that must read across multiple rows, yes, fewer pages are faster.
Above all that, the single biggest factor in overall performance is determining the best clustered index for every table. The next is to keep the statistics up to data. Index fragmentation / rebuilds are only third in priority for overall performance.