Quest performance is the main meter in quests in The Sims Medieval, and it represents the rank of the ongoing quest.
When a quest is completed, the player will be awarded with the same medal shown on the Quest Performance meter. There are four types of medals/ranks: Bronze, silver, gold and platinum. The higher rank achieved, the better reward will be given.
Any quest starts at the silver level, and depending on the controlled Sim's focus and quest completion, the meter can increase or decrease. Lowering it below the bronze rank will result in the Hero being arrested for not succeeding in achieving kingdom duties.
The Quest Performance will raise and lower over time, mainly based on Sim's focus. When the Sim's focus is green, it will raise, (the greener the faster); when it's red it will decrease (the redder the faster). It is important to note, though, that at full green focus it will increase slower than it will decrease when having full red focus.
The Quest Performance will also go down when the main quest is unattended for some time (i.e. more than a day), even with high focus. Sometimes if the action needed isn't crucial for the quest, after some time that part will be skipped and the next one shown. The player may want to be careful, though, it will still make the Quest Performance meter go down until a quest-related action is done.
Note that there isn't any indicator that shows how many quest actions the Sim has to do in the quest to complete it, so players who want to achieve platinum rank should avoid doing all the quest actions quickly. Building a high focus and letting the Quest Performance meter raise, while completing quest actions is the best way to proceed. Then when platinum rank is achieved, players can stick to quest actions in order to finish the quest.