The emulator and simulator execute only in debug mode.

Application performance can be janky in debug mode.
Measure performance in profile
mode on an actual device.

Release

Use release mode for deploying the app, when you want maximum
optimization and minimal footprint size. For mobile, release mode
(which is not supported on the simulator or emulator), means that:

Assertions are disabled.

Debugging information is stripped out.

Debugging is disabled.

Compilation is optimized for fast startup, fast execution,
and small package sizes.

Service extensions are disabled.

Release mode for a web app means that:

The build is minified and tree shaking has been performed.

The app is compiled with the dart2js compiler for
best performance.