About Cython and the main Sage commands

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I read the "How to import some commands?" and "Importing sage functions into cython?" topics, so I know how importing new commands in Sage and also under %cython.

I found that main commands like "factor" or "identity_matrix" which are automatically available in Sage are not under %cython. Then, it's necessary to add the lines:

"from sage.rings.arith import factor"
"from sage.matrix.constructor import identity_matrix"

Questions:

• Why the main commands are not automatically available under %cython ?
• Is there a line importing directly all the main commands under %cython ?
• Or, is it a good idea to improve the Sage software so that it works directly ?

I read also that such imported commands are not "cythonised" (and so not 100 times faster).

• Are there cythonised equivalents of all the main commands ?
• If not, is there a project to cythonised all the main commands ?
• Why Sage does not use directly Cython ?