Use the yeast the pros use.
Sweet breads, such as our Challah, can take more than twice as long to proof as unsweetened loaves (4½ hours versus the 2 hours typical for a standard loaf). In small quantities, sugar enhances fermentation and thus gas production. But because sugar is hygroscopic (meaning it attracts water), in high enough quantities it traps so much water from the dough that the yeast, which needs moisture to be activated, can't function as efficiently. Gas production then slows dramatically.
To counteract this phenomenon, professional bakers use a special strain of “osmotolerant” yeast that needs less water to function. When we made batches of challah and raisin bread with equal amounts of instant and osmotolerant yeast, we found that the osmotolerant yeast worked almost twice as fast as the instant yeast and produced results that were identical in flavor and texture. If you frequently make sweet yeasted breads such as challah, cinnamon rolls, or panettone, this yeast (available online) is worth purchasing. It's pricier than instant yeast but keeps for months in the refrigerator or freezer and can save you a lot of time in the long run.