DohertyThreshold

The maximum time a UI should take to respond to input from its user is 400ms.

In 1982, Walter Doherty and Ahrvind Thadani discovered that shortening computer response times to 400 milliseconds (from the accepted standard of 2 seconds) resulted in a disproportionately large improvement in productivity. When people no longer had to wait for the machine, they typed commands faster and spent much less time collecting their thoughts.

https://lawsofux.com/doherty-threshold/

See also: GaryBernhardt's FastTestSlowTest, in which he talks about the value of having a test suite that can run in under 300ms.