[solved] how to speed-up huge data in an ascii file ?

[Gribouillis]

The problem here is that you are not only saving the data to a file, you are creating the data. It is the data creation that takes time, not the writing.

You could perhaps play with the fmt keyword argument in numpy.savetxt, something like 'X=%.18e, Y=%.18e, Z=%.18e'.

import numpy as np
import sys
n=10 # 1_000_000# 10_000_000
M=np.random.random((n, 3))

np.savetxt(sys.stdout, M, fmt='X=%.18e, Y=%.18e, Z=%.18e')

Output:
X=1.778314452437265158e-01, Y=7.362842666045655848e-01, Z=8.358127207042234108e-01
X=5.591744788035918345e-01, Y=7.845951465943425962e-01, Z=6.039963855998189413e-01
X=8.327560563335355548e-01, Y=6.042091153798287984e-01, Z=1.590375469584719426e-01
X=9.855324666099820607e-01, Y=6.029884572061958714e-01, Z=3.114472999689985588e-01
X=7.433919307334269089e-01, Y=2.941350276294346644e-01, Z=6.780499010590056441e-01
X=7.791133512845780373e-01, Y=2.911042379946882086e-01, Z=8.546676365400691644e-01
X=2.481689914304145983e-01, Y=4.970687878118742464e-01, Z=1.684596602818245747e-01
X=5.134374653560572765e-01, Y=1.239447760698755285e-01, Z=3.211991817077095579e-01
X=6.812413908422187969e-02, Y=9.637812239995832142e-01, Z=8.384101532932353162e-01
X=7.187521467518211971e-01, Y=9.752591487821623550e-01, Z=1.938176050010664841e-01