This article describes a performance issue with drop down fields in Episerver forms. The more entries in the field the worse the performance.
This issue was identified and resolved in the AFORM-1448 here.
Please note that if the Marketing Automation Integration solution is being leveraged in the project updating to Episerver.Forms 4.9.1 was not possible.
This issue was resolved in AFORM-1456 which applied the same fix into Episerver.Forms 4.8.1. So if MAI is being used please apply that nuget package to have the fix.