[ubuntu] Slow boot up after upgrade from 14.04 to 16.04

Greetings all:
I just upgraded Ubuntu from 14.04 to 16.04, my system is as follows:
product: Intel(R) Core(TM) i3-3110M CPU @ 2.40GHz
vendor: Intel Corp.
physical id: 1
bus info: cpu@0
size: 1238MHz
capacity: 2400MHz
width: 64 bits

After the upgrade I got an "shim-signed" error, and also the boot time is higher than 14.04, how or what adjustment can I do to speed up the boot time?
ps: the shim-signed error was fixed by using the following instructions:
sudo apt-get purge grub*
sudo apt-get install grub-efi
sudo apt-get autoremove
sudo update-grub