# kitty Performance

The main goals for kitty performance are user perceived latency while typing and “smoothness” while scrolling as well as CPU usage. kitty tries hard to find an optimum balance for these. To that end it keeps a cache of each rendered glyph in video RAM so that font rendering is not a bottleneck. Interaction with child programs takes place in a separate thread from rendering, to improve smoothness.

There are two parameters you can tune to adjust the performance. `repaint_delay` and `input_delay`. These control the artificial delays introduced into the render loop to reduce CPU usage. See Performance tuning for details. See also the `sync_to_monitor` option to further decrease latency at the cost of some tearing while scrolling.

You can generate detailed per-function performance data using gperftools. Build kitty with make profile which will create an executable called kitty-profile. Run that and perform the task you want to analyse, for example, scrolling a large file with less. After you quit, function call statistics will be printed to stdout and you can use tools like kcachegrind for more detailed analysis.

Here are some CPU usage numbers for the task of scrolling a file continuously in less. The CPU usage is for the terminal process and X together and is measured using htop. The measurements are taken at the same font and window size for all terminals on a `Intel(R) Core(TM) i7-4820K CPU @ 3.70GHz` CPU with a `Advanced Micro Devices, Inc. [AMD/ATI] Cape Verde XT [Radeon HD 7770/8760 / R7 250X]` GPU.

Terminal | CPU usage (X + terminal)
--- | ---
kitty | 6 - 8%
xterm | 5 - 7% (but scrolling was extremely janky)
termite | 10 - 13%
urxvt | 12 - 14%
gnome-terminal | 15 - 17%
konsole | 29 - 31%

As you can see, kitty uses much less CPU than all terminals, except xterm, but its scrolling “smoothness” is much better than that of xterm (at least to my, admittedly biased, eyes).

Note

Some people have asked why kitty does not perform better than terminal XXX in the test of sinking large amounts of data, such as catting a large text file. The answer is because this is not a goal for kitty. kitty deliberately throttles input parsing and output rendering to minimize resource usage while still being able to sink output faster than any real world program can produce it. Reducing CPU usage, and hence battery drain while achieving instant response times and smooth scrolling to a human eye is a far more important goal.