GPicView

I'll recommend to you this Simple and Fast image viewer.

As you say that you computer is not a super fast calculator you should look for software with a few functions (the fewest possible), and this software meets this requirement, see the next screen-shoot and you will notice it.

You can install it through the Software-Center.

On my Xenial system gnome-system-monitor is fairly straightforward to build from source. First allow access to source code:

Dash > Software & Updates > Ubuntu Software >  Source Code

Then allow the system to reload the repositories. Pick up the build dependencies for gnome-system-monitor by running the following from within a Terminal window:

sudo apt-get build-dep gnome-system-monitor

Then download, compile and install gnome-system-monitor with the following single command which can be copied and then pasted into a Terminal window:

sudo apt-get install build-essential checkinstall && \
mkdir -v $HOME/Desktop/gnome-system-monitor_build && \
cd $HOME/Desktop/gnome-system-monitor_build && \
wget https://download.gnome.org/sources/gnome-system-monitor/3.20/gnome-system-monitor-3.20.1.tar.xz && \
tar xvf gnome-system-monitor-3.20.1.tar.xz && cd gnome-system-monitor-3.20.1 && \
./configure && make && \
sudo checkinstall --pakdir "$HOME/Desktop/gnome-system-monitor_build" \
     --backup=no --deldoc=yes --pkgname gnome-system-monitor --pkgversion 3.20.1 \
     --fstrans=no --deldesc=yes --delspec=yes --default

Log out and then back in (or simply open Dash, close it and then reopen it) and you should then see the following, (a screenshot taken from my own installation on Xenial Xerus):

Runs nicely on my system and when a new version is released this method can easily be tweaked to install the very latest version :)