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So the code you pasted is in an Uplink? Be aware that to actually add those data table rows Anvil has to make a call to the actual server. So you’re making 500 (or a 1,000) server calls in short succession. That’s bound to introduce some delays.
You might try bundling new rows as a list of dictionaries, and then make a server call that loops over that list and adds rows on the server. You’d need to play with how big that list of dictionaries could be, and do timings to see if there’s a sweet spot where you get the best performance. You’d at least be making fewer server calls than you are now, which should speed up the throughput.