Symptoms

You may notice significant quantity of network traffic in your network from SmartForms client machines towards SmartForms runtime site. Significant volume of traffic correlates with the loads of the following requests in the IIS logs for SmartForms site:

GET /Runtime/_trust/Login.aspx ReturnUrl=/Runtime/Runtime/Form/YOUR_FORM_NAME/Worklist.sdk 200

Volume of traffic as well as frequency of GET Worklist.sdk in K2 hostserver logs and the fact that these symptoms are in place even at the periods when user activity should be may raise some concerns/questions. In case you perform some sort of active network monitoring your network team may raise a question about this traffic.
 

Diagnoses

Frequent queries. Frequent queries mentioned above (GET Worklist.sdk) are coming from clients browsers. More specifically from forms where K2 Worklist control is placed. The frequency of this query for specific control is being controlled by configurable Worklist control refresh interval (default setting for it is 300 seconds or 5 minutes). So each instance of browser which has a form with this control opened in browser will be requesting new worklist data every 5 minutes. If you multiply this by number of active users who may have this form opened in their browsers (as well as the fact that they can have it opened in multiple browsers/tabs) you will get clear understanding about cause of frequency of GET Worklist.sdk queries in IIS logs.
The fact that this request is coming from control which performs refresh automatically every 5 minutes without user intervention explains why you may such queries even when users presumably supposed to be inactive. It is enough just to leave your browser with form open - it will generate such queries every 5 minutes by default.

High volume of traffic. When K2 performs GET Worklist.sdk it retrieves entire unfiltered worklisted from server and perform filtering on the client side. Essentially K2 pulls XML blob which contains all worklist items and perform filtering on the client side. This XML blob size may be quite large (e.g. 1,5 MB size depending on number of items in your worklist). If you multiply this by number of clients and refresh frequency you will get the cause of significant traffic being sent between SmartForms clients and IIS.
 

Resolution

If frequency of such queries is a problem then you can increase Worklist control refresh interval settings to decrease frequency of automatic refresh for this control.

For large volume of traffic problem there is an existing feature request logged to investigate opportunity to add pre-filtering of worklist on the server side in the future versions of K2 SmartForms.