How to fix 'C extension not loaded, training will be slow. Install a C compiler and reinstall gensim for fast training.'
I've faced this issue for a long time when I was running W2V Models which requires 'gensim'.
First of all I've installed Anaconda Navigator and then installed required packages using pip.
I've installed gensim manually using pip in cmd. When I run the W2V model, it took 40 min to train and give the result, which made me annoying and wasted a lot of time.
This problem got solved now. I just did what the warning showed. I've uninstalled gensim from my computer. Prior to that I've already created a system path of mingw-w64 in the environment variable which is an environment for c,c++ etc., programs. Later, I've reinstalled gensim using 'pip install gensim'.
Now the program is running within seconds which made a drastic change in the execution time.
Hope this helps...
I'm using the library node2vec, which is based on gensim word2vec model to encode nodes in an embedding space, but when i want to fit the word2vec object I get this warning:

C:\Users\lenovo\Anaconda3\lib\site-packages\gensim\models\base_any2vec.py:743:
  UserWarning: C extension not loaded, training will be slow. Install a
  C compiler and reinstall gensim for fast training.

Can any one help me to fix this issue please ?