On the Dashboard's Bottleneck tab, you can see how many invoices are with each user and how long the average processing time is.
The upper chart shows how many invoices are with each user in the review and approval stages. Via the button Send reminder mail above the bars, you can send an email directly to all people in the chart.
Click on one of the bars to get a list of the invoices the user has.
Above the list is the Send Reminder mail button, which is used to send a reminder directly to the current user.
Click on the button and you will receive an email form with the user's email address pre-filled, fill in the rest and click Send.
The chart below shows the users' average processing time over the past year and over the past six months.