learning-curve.jpg
This thing is called learning curve and it applies also to linguistics and it descripes how quickly we learn on the different phases/stages of learning.
At the beginning we learn very slowly because we have to learn the "rules of the game" (e.g. pronounciation, grammar etc) which do not have any relationship with actual communication. They are just mathematical exercises we should perform.
Afterwards we begin to learn the most usefull words (usually easy short words) which are used in the majority of discussions. For example the word говорить in comparison with the word Соловьи is used about 400 times more in the every day speech (except if you are ornithologist ). For this reason our understanding increases dramatically quickly (this is the middle part of the curve).We shouldn't forget that scholars claim that knowing only 3000 words we are able to understand the 90% of everyday discussions*. At the same time it is the excitement that we are entering a new culture. Also we shouldn't forget the power of repetion. Again using the example of the word говорить: we used it so many times that the use of its падежи becomes unconsiousness, we use it without even thinking of the mathematics.
Finally the speed we learn winds down because of: (a) complicated dictionary (i.e. complicated words that we come by very rarely i.e. if I see the word воссоединение just one time I will never learn it ) (b) accent and high speed of the way locals speak (it will take years to understand russian television and especially russian comedies) (c) structure of the language (d) idioms (e) slang.
The only way in my opinion to avoid stagnation is if you change the method of learning. E.g. start watching american films with russian subtitles.
* A local average russian (i.e. not the weirdos of Masterrusian who seem to know every single detail ) uses 20,000 words and recognises about 40,000. A foreigner who uses 10,000 if we consider that the additional 7,000 words are associated with his profession can even become dean of Московский государственный университет.
This is my opinion ... and allow me to repeat: RUSSIAN ARE KILLING ME!!!!