How light is Fenn?
Fenn sits on your screen all day, so what it costs matters. We measured it with the notch folded away and open, at start-up, under full CPU load, and with plugins, the taskbar and Hey Fenn turned on and off one at a time. These are the results, with the method at the end. Measured 12 November 2026.
Folded away, default settings.
Of the whole processor. No GPU work at all.
From launch to the notch on screen.
On a 144 Hz screen, even with every CPU core busy.
Folded away
Most of the day the notch is a small pill at the top of the screen. A few seconds after it folds away, Fenn hands its working memory back to Windows and waits. The pill's clock wakes it once a minute.
| Memory | CPU (whole processor) | GPU | |
|---|---|---|---|
| Default settings | 7.9 MB | 0.05% | 0% |
| With the OpenGL rendererSettings, Advanced | 6.9 MB | 0.02% | 0% |
| Heavy use16 plugins, Fenn's taskbar and Hey Fenn on. Fenn itself; plugins are below | 14.6 MB | 0.27% | 0.1% |
Memory is the working set, what Task Manager shows. While folded away Fenn wrote nothing to disk worth measuring, and its memory stayed flat over the whole run.
Open
Hovering opens the notch with a spring animation. Fenn draws at your screen's refresh rate while something moves, and stops drawing when nothing does.
- Each frame takes Fenn 0.6 ms at the median and 1.4 ms in the slowest 1%. At 144 Hz a frame has 6.9 ms, so Fenn uses about a tenth of it.
- 143 fps on a 144 Hz screen through opening, closing and switching pages. With all 16 CPU threads kept busy by other programs it still ran at 143 fps, with 92% of frames on time.
- Open and still, the tool grid with live tiles uses about 6% of one CPU core (0.4% of the processor) and 0.4% of the GPU.
Start-up
| Notch on screen | Closing | |
|---|---|---|
| Default settings | 0.4 s | 0.15 s |
| Heavy use16 plugins, taskbar, Hey Fenn | 0.55 s | 0.8 s |
Timed on a PC that had run Fenn before. The very first launch took 1.1 s while Windows loaded it from disk. When Fenn closes, it closes its plugins too: none were left running in any of our 26 launches.
Taskbar, Hey Fenn and plugins
Everything heavy in Fenn is something you turn on. This is what each one added while the notch was folded away, in CPU use per core (100% is one core fully busy):
- The taskbar costs less than 1% of one core and a few MB of memory.
- Hey Fenn listens for its name and recognizes speech on your PC, so it keeps its speech models ready: about 1.4 GB reserved with the Best models, most of it paged out while you aren't talking. For about 20 seconds after Fenn starts, it loads those models, which keeps several CPU cores busy. If you use a local Ollama model, Fenn asks Ollama to keep it loaded for 30 minutes so answers start quickly; that memory shows under Ollama, not Fenn. Turn Hey Fenn off in Settings and none of this happens.
- Plugins are separate programs, so each shows in Task Manager on its own. A typical Python plugin takes 15 to 40 MB and almost no CPU while idle; ones that run AI models take more (the largest we tested used 188 MB). Remove a plugin and its cost goes with it.
Storage
| Size | |
|---|---|
| DownloadSetup / Microsoft Store package | 11.2 / 13.5 MB |
| Installed | 37 MB |
| Settings and dataNew install / after months of use | 9 KB / about 3 MB |
| Hey Fenn speech modelsLight / Best, downloaded only if you turn it on | 206 MB / 1.05 GB |
| iPhone screen mirroringDownloaded the first time you use it | 254 MB |
| PluginsCode is small; some bring their own libraries or models | Under 1 MB to a few hundred MB |
Fenn keeps two log files and replaces the older one each time it starts, so logs never pile up. Search keeps a small file index (0.3 MB for a typical user folder). It indexed 142,000 files on a whole C: drive in a third of a second and finds a match in 1 to 3 ms.
How we measured
- Memory, CPU, threads and disk use were read once a second for Fenn and every program it started, for 1 to 5 minutes per condition. GPU memory and load came from Windows' GPU counters.
- Frame times come from Fenn's built-in frame log during a scripted 16-second run of opening, closing and switching pages, done with and without 16 busy threads loading every core.
- “Default settings” is a new install. “Heavy use” is a copy of a real profile with 16 plugins, Fenn's taskbar and Hey Fenn on. Usage stats were switched off for every test.
- This is one PC. A slower one will take longer to start and to load Hey Fenn's models; memory figures will be much the same.
See something different on your PC? Write to [email protected] with your Windows version and what you had turned on. For how Fenn handles your data, see the privacy policy.