In my experience, that Loop action is just incredibly slow and I avoid it whenever possible. One loop cycle will run then the workflow just sits there for several minutes before the next cycle kicks off. I don't understand what you're looping through here, but if there's any way you can use a For-Each instead of the loop I think you'll be much happier.

That's interesting. I did wonder if the loop was causing the performance issue. Basically, the UDA is called in the workflow and that decodes the XML chunk that the form repeating section produces. It then gets stored into a variable called textRepeatingSection, and another variable stores the number of child items in the repeater as an index.

A separate variable called numIndex is then set to 1 to begin the count of iterations through the loop. Then the loop continues to process while the number of children is greater than the count value, and stops when they are equal. In the loop, I just have an XML query, that uses XPath to recover the desired element values from the array stored in the textRepeatingSection variable and then the log action just outputs the values into the log. 

So, I guess the For-Each action needs to approximate what the looping is doing. I will see if I can reconfigure that part of the workflow to see if the performance improves.

I read in another post, that using the internal FormData in the UDA is less reliable than using the list lookup to recover the XML chunk for processing. I will look at that, once I have the looping working better. Thanks for your suggestion.

I suspect that the reason this is happening is because you have Safe Looping turned on

To quote the article: "Enforce safe looping (enabled by default): Automatically adds a hidden delay at the end of each iteration of a loop action (Loop only) if the child actions are not guaranteed to cause the workflow to stop and wait. This delay can cause a 2- to 7-minute pause between each run of the loop. Delays are added when the workflow is published; to see the change in behavior, republish existing workflows that use loop actions."

Additionally: "For Nintex Workflow for SharePoint 2016, safe looping can be enabled/disabled via Central Administration > Nintex Administration > Security"

Turning this off should solve your problem without needing to rework anything. 

OMG! It's nobbled by design! Thanks @MegaJerk for this solution. I removed the safe looping option and (after a 15 minute wait), republished the workflow and ran it on an item with 5 repeats. The workflow finished instantly. 

Ironically, I had also redesigned another workflow to use an XML query into a collection, and then within a for-each loop, I recovered all the other fields using collection operations set to Get. Here is my alternative solution, which does not use the UDA...

It works just as well, and I picked up some threads on this board, that suggested that UDAs will not be available, or may not work in the Online versions? Nevertheless, I am happy that both solutions now work. Thanks also to @jeff for his suggestion of the For-Each loop. That has worked well.

@SmashedMonkey instead of using loop, the most convenient method to extract information from repeating section is as below:

1. Create a plain text MLT column in SharePoint



2. Connect your repeating section to this MLT



3. In your workflow, using Query XML and above MLT column as XML source, extract all the field values you need and store them in a collection variable



4. Using any one collection variable, for loop and Index variable
   
   
   
   
   
   1. Now for each collection variable use respective collection operation and using index variable from for loop get the value and store it in the respective type of workflow variable
   
   
   
   
   
   

This way no need of:

1. Publishing UDA



2. Safe Looping



3. Waiting on a timer job

Just my 2 cents.