Inventory Valuation Slower than normal

We are using NAV 2013 and our inventory valuation is now running slower than normal. Nothing has changed on the server and it is doing it on multiple workstations. Any Ideas what could be doing this?

You say it’s doing it on multiple workstations. But not on all?
If nothing has changed on the server/hardware, then it could either be change in the software (any NAV upgrades/customizations installed?) or the data. More data?