Does pre-populated body for content types in Drupal 7 affect performance too?
tl;dr: No.

What happened in D6 when you shared a field, was that it got moved to it's own table.
This became irrelevant with D7, as all fields are now stored in separate tables, which was a very controversial decision, due to the performance effects this has.

If you want to increase performance, you can read some more here: How do you improve drupal performance.