This is due to frecon resetting scrollback buffer position after every keypress which is reasonable, because terminal should go back to end of scrollback buffer in such case.

libtsm does force redraw on every reset, even though scrollback buffer position does not change. So the screen is redrawn on each keypress even though it does not have to be.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/b59468b0ea8448a3742faf8ea642e8053b4883a4

commit b59468b0ea8448a3742faf8ea642e8053b4883a4
Author: Dominik Behr <dbehr@chromium.org>
Date: Fri Oct 28 23:55:37 2016

libtsm: do not reset scrollback buffer position if it has been already reset

BUG= chromium:660602 
TEST=draw full screen rectangle on frecon VT and then press any key

Change-Id: I590328a1011ddfbb24057aa937c52326a5dd5ab0
Signed-off-by: Dominik Behr <dbehr@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/405107
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>

[rename] https://crrev.com/b59468b0ea8448a3742faf8ea642e8053b4883a4/sys-apps/libtsm/libtsm-3-r2.ebuild
[add] https://crrev.com/b59468b0ea8448a3742faf8ea642e8053b4883a4/sys-apps/libtsm/files/0002-libtsm-do-not-reset-scrollback-position-and-age-if-i.patch