Calculating the ascent (and hence the Naismith time) in our GPS mapping application is very costly in terms of server time and we were finding it was starting to impact on other users of the server when a user was adding waypoints to a long route1 (as the software has to keep re-calculating the total ascent over increasingly long distances). So we've now limited the automatic re-calculation to routes of 30km or less.
If your route is more than 30km then you will see a "Calculate" hyperlink in place of the ascent and Naismith figures in the "Your Route" box thus:
Click on either "Calculate" and it will re-calculate the figures for you.
We picked 30km because most day hikes will be less than that (unless you're very fit!) but the "calculate" option still means you can calculate total ascent when planning longer routes e.g. multi-day hikes or cycle trips.