When you have a form and you refer to the smartobject, you can put limits on the lookup by filtering from a different column.   I have a sharepoint list that  has 4k+ items in it, and i processes it just fine.  There is a small delay due to the download onto the form to write the information.

Thanks, yea i have used filters.  For my use case i am picking up a column from a parent list to use.  That column needs to bring in just over 3000 items so that the user can pick a single selection. The drop down has recently started cutting off options, as if there is a limit to the amount the lookup can retrieve. 

This was resolved with the settings for the lookup limit in the Responsive designer.  So we can get hit 5k rows. Thanks!

I have lookups to lists with over 25,000 items. It is a bit slow, but it works. Using filters definitely speeds things up. However, if you want it to work really fast, do away with the drop-down if you can.

For a number of forms that I have, it contains information about our product parts. For most forms people know what the part number is. When the part number is entered, create a rule with the Lookup (nintex.com) function to the list to populate any other data (in my example, part description, unit price, category, export restriction, etc.).

Also, if you are looking for solution to sync your backend system data with a SharePoint list, Data Integration & File Sync for SharePoint and Office 365 | Layer2 Cloud Connector (layer2solutions.com) is great for this. This is what we use to keep our parts list updated.