The phrase like ᴠᴇʀʙing through molasses (AmE) or ... through treacle (BrE) is commonly employed to convey the idea of very slow progress.
Typically, this is not due to active resistance, rather it is used reflectively, as a observation made in hindsight. The analogy is possible, as @HotLicks points out, due to the high viscosity of fluids like molasses, treacle and honey.
Alternatively, another common versions of this idiom is ‘wading through treacle’, this thick sticky black (or golden) syrup will be unfamiliar with many American English speakers. But if you're a fan of Harry Potter, you ought to know that his favourite dessert is treacle tart. Google Books reports 1,820 results for the idiom.
wade through (Cambridge Dictionaries)
to spend a lot of time and effort doing something boring or difficult, especially reading a lot of information: