Is feeling whispers (waswas) such as belittling people, being arrogant towards them, and being conceited after performing acts of worship, an indication of an unsound heart?

The difference lies between blameworthy self-admiration (ujb), which is seeing oneself as the source of favor and forgetting the Bestower, and the praiseworthy joy in Allah's blessings and grace, feeling His tremendous bounty. It also lies between mere fleeting thoughts and whisperings (wasawis) and actual heartfelt actions. If whisperings recur, one is advised to supplicate, humble oneself, distract oneself from them, and consult a trustworthy psychiatrist.