Processing efficacy is based on the idea that objects differ in the fluency with which they can be processed. Our judgement of something can be dramatically altered by how fluent it seems to process it and we engage more positively with high fluency experiences. Fluent processing can be facilitated by several variables such as repeated exposure to a stimulus, aesthetic attractiveness of the object, expressions that rhyme, and so on. By contrast, low processing efficacy occurs when we find something difficult to interact with or understand and so it requires more cognitive effort and strain, which results in a negative feeling towards it.
For example, several experiments have revealed that people are more likely to react positively towards, and agree with, statements that are easier to read: the lack of cognitive strain involved with comprehending the statement results in an intrinsic positive feeling towards it and simplicity is also translated as beauty in the human mind and we often judge something we perceive to be more beautiful as more positive and truthful.
Processing efficacy has multiple applications in web-marketing, especially with regards to website design: the aesthetic attractiveness, the page speed load time or the ease of interaction of your website are all factors that will affect whether your visitors enjoy using your website and therefore engage and interact with it, complete actions, share it on social media, recommend it, etc.