If your belt is too tight you can slightly dampen it with a barely wet towel while gently treadling. Like tight leather shoes, remember ?
Right foot upper right corner and left foot lower left corner. If there is a lag it may not be the belt but the treadle. Does it clank? It could need adjusting.
When tightening your belt just gently unfasten the link after throwing your belt. Be sure when handling the belt not to have greasy or lotioned hands. Slippage. Keep the belt to the right of the band wheel for easier realigning. Now holding the belt firmly, put it together end to end while it is in place like you would sew. Move the belt ends together and pass them so that you get an idea of how much to trim. Mark with your chalk. Cut. Use your stiletto or belt tool to make your hole, refasten the link using two pairs of pliers or your belt tool. Remember to keep the belt to the right? Ok. Place the belt over the fly wheel and adjust it. Gently and carefully hold the belt against the band wheel and pull the band wheel toward you slowly. The band wheel has a tab that will catch the belt and place it perfectly in line.
Hope that helps.