That test runs much faster today:
✓ should type 36 barcode characters (3765ms)
✓ should insert 36 barcode characters (1755ms)
but if I type as fast as possible, it feels like there's a debounce on the field. There is literally no visible change to the field until I stop typing, and then the whole string appears. If I type quickly but not too quickly, with a short but definite pause between each stroke, the debounce feeling goes away; the input is smooth.

Idle speculation: are we running redux-form validation functions via onChange?

Definitely warrants further investigation.

Yes, this is still an issue.
The automated test posted on 29/Jun/18 is still slow on http://folio-testing.aws.indexdata.com :
√ should type 36 barcode characters (19436ms)
√ should insert 36 barcode characters (34280ms)

Is this still an issue? There's a chance https://github.com/folio-org/stripes-components/pull/598 resolved it. We'll investigate further if still occurring.

I've created an automated test to show how slow it is:
https://github.com/folio-org/ui-users/blob/STCOM-287-entering-characters-slow/test/ui-testing/type_barcode.js

How to run:
cd ui-testing
export FOLIO_UI_URL=http://folio-testing.aws.indexdata.com
yarn test-module -o --show --run=users:type_barcode

The result for http://folio-testing.aws.indexdata.com is:
√ should type 36 barcode characters (16890ms)
√ should insert 36 barcode characters (33152ms)

The result for http://folio-snapshot-stable.aws.indexdata.com is much faster but also too slow:
√ should type 36 barcode characters (3797ms)
√ should insert 36 barcode characters (5296ms)

The textfield is still as slow as shown on the video of 2018-06-04.
The textfield is also slow on Chrome and Firefox when the browser runs on a Ubuntu machine. The faster the machine the faster the input.