No kernel change. Basically changes to /etc/default/grub are used when
running update-grub to create the grub configuration. Leaving
GRUB_TERM=console commented out causes grub to initialize some higher
resolution graphics framebuffer mode. And (not quite sure) it might be
that about some point in time this changed to be a higher resolution.

But looking at the two dmesg outputs you posted (possibly you have both
kernels installed on the same box, so grub setup is the same in both
cases just the newer kernel ends up being slower) shows both vesafb get
initialized nearly identical.  So right now its hard to say why things
go so much slower in the newer kernels. The simple framebuffer imo
should not be so much different in how it works. Cannot say why that
works faster. simplefb might just be one of them. There is a whole bunch
blacklisted in /etc/modprobe.d/blacklist-framebuffer.conf because they
tend to cause issues.

One other detail I noticed in the dmesg output was that the newer kernel
managed to initialize local ipmi drivers. Not sure how much this is
actually needed. As long as its not required to have access to ipmi
directly (not lan) one can disable ipmi_si and dev_ipmi can be disabled
in /etc/default/openipmi. Just mentioning that since console usually can
be accessed via ipmi and maybe there is some kind of interference.

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https://bugs.launchpad.net/bugs/1434581

Title:
  Console extremely slow with 3.13 and newer kernels for certified
  servers with Matrox G200er2 or similar

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