I am using survey123 version 3.5.164. I am using bar code in my forms. When using bar code in android the camera to check the bar code becomes very slow. I have tested on iphone 6 and it works good. This problem didn't occur in previous versions. Note that I am using S8 and Samsung tablet Galaxy S2, S3 , and note 9.
Can you confirm you are still having barcode scanning issues with latest version of Survey123 field app (3.10.326). In recent updates there have been a lot of improvements to barcode scanning.