RE:
"I don't really know why this happened but recently my default Calculator starts very slowly. Heck Firefox starts faster. What is causing this? Must I reinstall that particular app?"
Is it an "Intel-native" calculator that came with the system software, or could it be an older app that isn't Intel-native? If it's older, that could explain the longer startup time (due to processing of older code into Intel code via Rosetta).
Another thought:
See if it has a related "prefs file". Try deleting the prefs file, then re-launch it.
- John