1) Go into Settings -> Performance.
1.1) Toggle the setting "Progress Bars" to either "Performance Bars" or "No Progress Bars" (Personal preference, as long as it is not on "Progress Bars"). This is the most important setting to take care of.
1.2) Toggle "Queue Animation" to "No Queue Animation"
1.3) Toggle "Fading" to "Not Fading"
1.4) Toggle "Heirloom Animations" to "No Heirloom Animations". If you don't see this setting, you may need to progress further in the game to unlock it.
1.5) Toggle "Generator Animation On" to "Generator Animation Off". If you don't see this setting, you may need to progress further in the game to unlock it.

Should look like this:

2) When AFK, put the game in a big menu, such as the Achievement screen, or the View Perks screen. This effectively kills the game's renderer and should halve CPU otherwise spent on all the screen updates. Works kind of like "AFK Mode" in similar games.

3) Make sure you do not leave the Steam Overlay open when AFK. The game increases its framerate while the overlay is open to make it possible to use the overlay, which comes at increased performance cost from the game.

The performance setting that used to be outlined here was added in an update to the Settings menu, called Generator Animations! You don't have to do any more things (that we know of) currently. I will keep the below explanation for posterity though:

Due to the way that rendering works on the web, Trimps by design runs on an inconsistent framerate, based on how often things are changing on screen. Trimps runs a logic update every 100 milliseconds, meaning that for best performance, we want the game to continue running at little over 10 FPS. Of course when actively hovering over stuff, the framerate may spike higher, but that is unavoidable.

Now, when things are animated smoothly on screen, the game is forced into a higher framerate, thus increasing CPU load. If you have already toggled all the options outlined in the previous section, you will have turned off all animations (that I know of).