Research with children has demonstrated that the ability to learn new words is greatly affected by working memory span - specifically, by how much information they can hold in that part of working memory called "phonological short-term memory". The constraining effect of working memory capacity on the ability to learn new words appears to continue into adolescence.
But, as you grow in experience, building a vocabulary, this constraint becomes less important. Because working memory capacity is measured in "chunks" - and the amount of information contained in a chunk is extremely malleable. To a large extent, developing chunking strategies is what memory improvement is all about.
In terms of learning another language, there are essentially four possible classes of word:
- words that are already familiar because they are the same in your native language (or another known language)
- words that are already familiar because they involve words that you already know in that language (e.g., learning a related verb form, or learning a word made up of two words you already know, such as sweat-shirt)
- words that resemble a known word with similar or related meaning (e.g., Russian garlo means throat, and the word garlo resembles the word gargle)
- words that have no ready association to known words
It appears that in these first three cases, the size of your phonological short-term memory is of no significant relevance. It is only in the last case - where the word cannot utilize any meaningful associations - that your phonological short-term memory capacity becomes important.
Fairly obviously, as your knowledge of language (your own and others) grows, the more meaningful associations you will be able to make, and the fewer new words will fall into this last, difficult, category.
This suggests, of course, the usefulness of a mnemonic strategy (specifically, the keyword strategy) in the last, difficult case.
The importance of phonological short-term memory is also greater for productive learning (learning to produce a language, i.e., speak or write it) than in receptive learning (learning to read or understand a language). For productive learning, the pronounceability of the new words is very important. The more easily pronounced, the more easily learnt.