As a brief note, I recently watched an episode of Death in Paradise where a murderer put salt in a candle to make the candle burn slower. Out of curiosity to see whether or not this would work, I burned a small candle without salt, and found that it lasted about ninety minutes. I then added some salt to a second candle of the same size, and it burned most of the day(!). One trick of this is that the candle has to be at a point where it’s already melted a little bit, so you’re pouring the salt into the melted candle wax.
Over time I repeated this test a few times, and sure it enough, adding salt to a candle makes it burn slower. So, if you watched that Death in Paradise episode and were curious about this, I can confirm that adding salt to a candle does make it burn slower.