All scales are going to have some amount of delay as the weight settles and stabilizes on the scale although how bad the delay is will vary. We have this scale for measuring smaller amounts of resin and silicon and I have this scale for measuring fabric dyes and small amounts of threads/beads/string/etc. Both scales have a tiny delay but it's generally only a fraction of a second unless I'm measuring something that takes a bit longer to settle. Personally accuracy is more important than speed so I'm ok with the scale taking a few seconds if the readings are going to be more accurate.

As far as accuracy get a set of calibration weights so that you can verify the accuracy of whatever scale you are using.