going from memory here, so I may be a little rusty.
Firstly I found greater success scanning with higher resolution than with lower, I have no explanation as to why.
Secondly, I always just stored the barcode data to a text field rather than a container, i.e. the decoded barcode info was only stored as text.
Thirdly, I found the whole process of scanning multiple barcodes quickly to be a PIA using the iPads camera regardless of any layout I could design to make the whole process seamless. I ended up investing in a number of Symbol CS3000 series bluetooth scanners that pair with the iPads and I can control the "flow" with scripts. Much quicker and simpler than using the inbuilt. One piece of advice, if you go down that road ... create a backup layout they can use when they can't find the BT scanner :), it'll remind them how laborious it is to use the conventional in camera method compared to the BT scanner and they will be more inclined to look after the scanner.
I've actually been using a bluetooth scanner for some time attached (literally with velcro) to my iPod Touches - they're symbol LS4278 scanners - available on e-bay for about $50 each. The problems I've had are 1) battery life of two independent devices isn't always ideal, and 2) I've found there has been a certain level of corrupt data between the devices. My belief is that when I hand the device to a novice user, they either hear the beep from the scanner indicating a good barcode read and move on assuming the record was written properly, or that if I disable the beep, they just move too fast for FileMaker to keep up with the data. What ends up happening is when I look back at the records captured, I get scans now and then that are missing a digit - which indicates to me the iPod can't keep up with the scanning speed of the external scanner - that's why I'm leaning towards a more integrated approach where the scanner is either physically attached like a LineaPro, or by attempting to use the onboard camera - but I fear as I've indicated that the camera is going to be too slow for my needs.
Yes, there is a latency as the buffers transmit data and depending on how big the code you're scanning will impact on that latency. In my own case I control the record flow with scripts and relationships, we scan EAN128 codes with many AI identifiers. I am scanning into a stock movement table which is a portal on a form. There is a further relationship between the stock movement table and the products table based on the Outer case 14 digit barcode. Basically the user scans the whole 128 barcode, into a scandata field, an on object validate script trigger strips out the 14 digit outer case code and updates the stock movement record barcode field, if the relationship finds 0 or more than 1 record in the products table I revert the record and inform the user there was an invalid scan or the product can't be found. But if it finds only 1 record in the products table I assume I have valid barcode scan and I strip the rest of the data I need from the scandata field and update the relevant fields, then I script movement to the quantity field and update it by 1 then move to a new portal row. The user can always go back to the quantity field and adjust the quantity if needed.
I would think we are scanning at a rate of 1 item per 1.5 seconds on a server hosted solution with these symbol scanners and iPads. I am aware the user can make the scanner scan faster than the scanner updates the iPad and without the check above we would have corrupt data.
Another solution to consider if it is an option is the internal batch scanning function on the scanner if it has one. That option didn't really suit my needs.
Our symbol scanners cost considerably more than $50, but they are used 8 hours a day between charges and do a lot of work, it might be worth trying one, they are also a lot smaller than yours.
Barcode scanning by iOS device is best for low volume--few at a time scans. The main delay is in waiting for the camera to bring the barcode image into focus. Sometimes, you are at just the right distance, the focus happens nearly instantly and other times you have to wait for the focus and maybe adjust the distance between device and bar code.
But Insert From Device does work as long as you are not in a hurry and the process can be scripted.