Metabolism and Hofmann elimination 
Deacetylating vecuronium at position 3 results in a very active metabolite.  In the case of rapacuronium the 3-deacylated metabolite is even more potent than rapacuronium. As long as the D-ring acetylcholine moiety is unchanged they retain their muscle relaxing effect. Mono-quaternary aminosteroids produced with deacylation in position 17 on the other hand are generally weak muscle relaxants. In the development of atracurium the main idea was to make use of Hofmann elimination of the muscle relaxant in vivo. When working with bisbenzyl-isoquinolinium types of molecules, inserting proper features into the molecule such as an appropriate electron withdrawing group then Hofmann elimination should occur at conditions in vivo. Atracurium, the resulting molecule, breaks down spontaneously in the body to inactive compounds and being especially useful in patients with kidney or liver failure. Cis-atracurium is very similar to atracurium except it is more potent and has a weaker tendency to cause histamine release.
Structure relations to onset time
The effect of structure on the onset of action is not very well known except that the time of onset appears inversely related to potency. In general mono-quaternary aminosteroids are faster than bis-quaternary compounds, which means they are also of lower potency. A possible explanation for this effect is that drug delivery and receptor binding are of a different timescale. Weaker muscle relaxants are given in larger doses so more molecules in the central compartment must diffuse into the effect compartment, which is the space within the mouth of the receptor, of the body. After delivery to the effect compartment then all molecules act quickly. Therapeutically this relationship is very inconvenient because low potency, often meaning low specificity can decrease the safety margin thus increasing the chances of side-effects. In addition, even though low potency usually accelerates onset of action, it does not guaranty a fast onset. Gallamine, for example, is weak and slow. When fast onset is necessary then succinylcholine or rocuronium are usually preferable.
Elimination
Muscle relaxants can have very different metabolic pathways and it is important that the drug does not accumulate if certain elimination pathways are not active, for example in kidney failure.