Reduce device and environment variance
Limit common sources of variation before comparing reaction, memory, targeting, or typing runs.
Control the input method
A mouse, trackpad, touch screen, and software keyboard introduce different movement and response delays.
Use one input method for a series you want to compare.
Reduce browser load
Close distracting tabs and wait for heavy downloads or updates to finish before a timing-sensitive test.
Keep browser zoom and display settings consistent for visual tasks.
Note meaningful changes
If you change hardware, browser, language, or environment, begin a fresh comparison period instead of assuming the old baseline is identical.