(as) slow as molasses

Examples from literature

As the juice is cooked, sugar crystals can be seen in the molasses. 
In factories there, rum was made from molasses, and cloth was made from cotton. 
Molasses has things in it that are very good for our health, but refined sugar does not. 

molasses | American Dictionary

molasses
noun[ U ]
us/məˈlæs·ɪz/
a thick, dark, sweet liquid made from sugar plants

Examples of molasses

molasses
And raw sugar, alcohol and molasses are not only exported but sustain important national industries in brewing, distilling, canning and confectionary.
Another group of seven goats received only the mixture of molasses and oat grains as a placebo.
The high grain ration initially comprised 50 % rolled sorghum and 50 % feedlot meal mix (lucerne, sorghum, barley, molasses, macronutrients, no urea or antibiotics).
An additional 14 mills would produce only molasses or other cane by-products.
Chlamydospores were mixed with molasses and oat grains as previously described.
Experimental animals were trained to eat the oat/sugar cane molasses mix, which was used as a vehicle for the chlamydospore treatment as described above.
Cane-tops, leaves and molasses, in crude or processed form, are important sources of animal foodstuffs.
Micro-organisms were multiplied by transferring 50 liters of the raw material to another tank (500 liters), where 20 liters of sugarcane molasses were added to 430 liters of water.
Cassava or sweet potato forage as combined sources of protein and roughage in molasses based diets: effect of supplementation with soybean meal.
Finally, citric acid is made from molasses, which is an uncontrolled commodity and cheaper than world price sugar.
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