Because you are using the call web service, there are other factors that may be affecting the execution time of the call. I would suggest testing the call in a service like postman to see the response and then removing the actions to test how fast the workflow executes without that.

Then see if that is something that can be increased. 

Just tripped over this type of post and found this solution helpful mcdvoice though it is old. Thanks for the post 

To check for any list/library assumed settings/numbers for List View related Thresholds:

1. Log on to the SharePoint Administration Portal.



2. Click “Application Management”.



3. Under “Web Applications” click the “Manage Web Applications”.



4. Select/Click the desired Web Application and click the “Resource Throttling” in the context menu of the “General Settings” button in the ribbon mcdvoice



5. Check if the configured settings are within Microsoft’s recommended values.



6. If not, investigate the reason for the changes and if it was tested and validated prior the change.

To check for any list/library assumed settings/numbers:

1. Log on to the SharePoint Portal;



2. Click “Site Contents” and check the item count for the intended lists/libraries;



3. Confirm if the existing values are within Microsoft’s recommendations or execute Microsoft’s recommendations to deal with large lists (