When I updated to kernel-default 4.4.155-68 (sep 2018)
the screen took such a long time, about 4s, to repaint itself,
that I could hardly use X.

I waited for the next update, that was 4.4.159-73 (oct 2018),
but with it the same problem occured.

So I have been downgrading to 4.4.143-65-default (by editing grub configuration
files) ever since.

Output of ``sudo /sbin/lspci|grep -i graphic`` is :
08:00.0 VGA compatible controller: ASPEED Technology, Inc. ASPEED Graphics
Family (rev 30)

I forgot how to get more information on graphics cards.