Some disagree about the terminology, as should be expected, but semantic value is the distinction between an 'interfix' and an 'infix' in English.
An 'interfix' is
(linguistics) An empty morph inserted between two morphemes in the process of word formation, such as English -o-, -i-.
[interfix. (n.d.). Retrieved December 16th, 2015, from http://www.yourdictionary.com/interfix.]
For 'interfixes' the inserted morph has only a phonological value. Such morphs are represented in speedometer and humaniform.
An 'infix', also called an 'integrated adjective', has semantic value as denoted by the alternative term. 'Integrated adjectives' modify the semantics of the associated nouns, usually but not always by intensifying them:
An infix is a word element (a type of affix) that can be inserted within the base form of a word (rather than at its beginning or end) to create a new word or intensify meaning. Also called an integrated adjective.
(From "infix (words and grammar): Glossary of Grammatical and Rhetorical Terms" at About education.)
Examples of infixes include anyoldhow, absobloodylutely.
As quoted at the latter source, R.L. Trask in The Penguin Dictionary of English Grammar (2000) points out that the closest thing to a 'true' infix in English is the pluralizing -s in compounds:
English has no true infixes, but the plural suffix -s behaves something like an infix in unusual plurals like passers-by and mothers-in-law.
Trask's justification for excluding 'integrated adjectives' from the class of "true infixes", as well as his justification for excluding the plurals of compounds, are not readily ascertained from online sources.
The central disagreement about the terminology stems, again as might be expected, from a technical point. Hairs are split about the terms 'interfix', 'infix' and 'circumfix':
Technically, it also possible to have an infix (added in the middle of a stem), an interfix (in between two stems) and a circumfix (added on either side of a stem), but these are extremely rare in English.
(From The History of English, L. Mastin, 2011. Emphasis mine.)
By this definition, 'infix' is exampled by absobloodylutely, 'interfix' is exampled by anyoldhow, and 'circumfix' is exampled by unconsciousness.
The definition proposed suggests that 'integrated adjective' is not an sufficient alternative term for 'infix', and defines 'interfix' in contrast to 'infix' by morphological features rather than the presence or absence of semantic value.