The free space is added to the pie chart if the root of the scanned drive is selected and the according option is activated in the context menu of the pie chart. In other situations (e.g. if a sub-folder is selected or the "Number of files" mode is active), it doesn't make sense to add the free space because the relationship between the free space and the other values displayed is wrong.
In the context menu of the topmost folder you may choose to hide folders below a certain size. To achieve this, right-click on the topmost icon (or use the "Expand" button on the ribbon bar), select "Expand > Hide Folders smaller than: XX MB".
Scanning a drive via network is normally much slower than scanning a local disk because speed of the network is much lower, latency and overhead are higher compared to a local hard disk. Additionally, server drives are usually much bigger and contain more files than local hard disks. But you can use scheduled Scans to perform your scans overnight. Network drives and command line options are supported by the Professional Edition only. For continuous analyzing of disk usage on large servers we recommend our product SpaceObServer. It collects the data using a background agent and stores it in a SQL database. It uses less RAM than TreeSize, and it offers more flexible reporting capabilities like historical development because it is built on a database. More information is available at: https://www.jam-software.com/spaceobserver/