# Why I am facing an issue where writing to the PLC tags happens quickly, but reading the tag values has a very big delay **URL:** \ **Category:** Ignition\ **Tags:** perspective\ **Created:** [December 4, 2025, 12:11pm UTC](https://forum.inductiveautomation.com/t/why-i-am-facing-an-issue-where-writing-to-the-plc-tags-happens-quickly-but-reading-the-tag-values-has-a-very-big-delay/112613 "2025-12-04T12:11:46Z")\ **Posts on this page:** 1\ **Showing post:** 3
**Author:** ![nminchin](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/nminchin/32/6911_2.png) [@nminchin](https://forum.inductiveautomation.com/u/nminchin)\ **Post date:** [December 4, 2025, 1:19pm UTC](https://forum.inductiveautomation.com/t/why-i-am-facing-an-issue-where-writing-to-the-plc-tags-happens-quickly-but-reading-the-tag-values-has-a-very-big-delay/112613/3 "2025-12-04T13:19:26Z")
I’m guessing this is an AB plc? Logix driver? If so, are you reading AOI parameters? (2000 requests suggests so) These will kill performance. Read UDT members or AOI locals making sure each item within is set to have at least Read ext. Access You hopefully didn't just drag over all of the tags from the opc browser into your project, but it’s certainly been done before… definitely don’t do that if you did. > [@Ignition & Logix Best Practice Tips for better communications](https://forum.inductiveautomation.com/t/ignition-logix-best-practice-tips-for-better-communications/60386/42): > > I recently decided it was time to actually test the theories with scan rates for different structures in the PLC, and found the results below. What I found (obviously) backed Phil's recommendations to only read UDT tags and to ensure that all members of them are set to at least read access. However what I found was negligible difference in response rate at 100ms when reading the UDT tags from within different structures (e.g. individual UDT instances at the root/controller level, addressing UDT… Also, set the cip connection size to 4000 for capable hardware. > [@Ignition & Logix Best Practice Tips for better communications](https://forum.inductiveautomation.com/t/ignition-logix-best-practice-tips-for-better-communications/60386/52): > > I always forget about this... Holy moly Edit: Updated results with tests using different CIP Connection sizes: Test Ext Access CIP Connection Size Tag Count Sample Rate (ms) Actual Sample Rate (ms) Request Count Overload UDTs All Read/Write 500 6787 10 45.48 14 355% UDTs All Read/Write 4000 6787 10 10.06 2 1% AOI Local Tags (UDT) All Read/Write 500 6787 10 50.86 14 409% AOI Local Tags (UDT) All Read/Write 4000 6787 10 10.08 2 1% AOI Parameter Tags All Rea… --- _[View the full topic](https://forum.inductiveautomation.com/t/why-i-am-facing-an-issue-where-writing-to-the-plc-tags-happens-quickly-but-reading-the-tag-values-has-a-very-big-delay/112613)._