Ubuntu – Why does the CPU usage keep spiking

Oh, boy, I just caught this... quoting OP:

Furthermore, even after the process ends, the computer does not return to the previous performance. I found a way around this by running sudo swapoff -a followed by sudo swapon -a

OK, so that means you were exhausting the available RAM on your system, which means you're just plain trying to run too many convert processes at once. We'd need to look at your actual syntax in spawning convert to advise, but basically, you need to make sure you don't try to open more simultaneous processes than you have the RAM to comfortably handle.

Since you state what's causing this is convert *.tif blah.pdf, what's happening is that the content of every single TIF and its conversion to PDF are getting stuffed into RAM at once. What you need to do is split the job up so that this isn't necessary. One possibility that leaps to mind is instead doing something like find . -iname '*.tif' | xargs -I% convert % %.pdf, then using pdftk or something like it to glue all the individual pdfs together. If you really want to get fancy, and you have a multicore CPU, this also affords you the chance to write a small script to run conversions in batches of n, where n is your number of cores, and get the whole thing done significantly faster. :)

pdftk how-to: http://ubuntuhowtos.com/howtos/merge_pdf_files (basically boils down to sudo apt-get install pdftk; pdftk *.pdf cat output merged.pdf)

This is a bug but here is a workaround until there is a bug fix. Close the psensor graph window instead of minimizing it. When you want to view the psensor graph window again right click the little psensor thermometer icon in the panel (top bar), click "Show" and then click the psensor icon on the Launcher (left sidebar) to show and put focus on the psensor graph window.

The spike in CPU usage doesn't only occur when the psensor graph window is minimized but anytime the graph window does not have the focus. You can verify this by having just one other small window on the desktop, Terminal for instance, and Alt-Tab between them. You'll see the CPU usage jump and fall as you do.

This is a bug with psensor 1.1.3-2ubuntu3 in ubuntu 16.04. The jump in CPU usage does not occur in an identically configured psensor 1.1.3-2ubuntu2 in ubuntu 15.10.

I'm running a custom built PC with an AMD A8-3870 APU and no discrete graphics card or motherboard graphics. I am not using proprietary graphics drivers just the default graphics with ubuntu 15.10 and 16.04.