There seem to be two processes which are being confused in this thread, manual sign in/out on the console and automatically using barcodes.
Staff, and I assume Sixth Form (we don't use ours for students), manually signing in or out on the console touchscreen takes between 10 to 30 seconds: Press Staff, Press first initial, press your name, choose reason for signing out (nothing here for signing in), a few seconds for the screen to say " signed in", etc.
Visitors are prompted to type their name, who they are visiting, car reg, then take a picture (5, 4, ..., click), are you happy with this?; this obviously takes longer than staff. As they type in their name, it prompts them with people who have visited before, and if they choose one, it offers them the choices from last time as a default. It always takes a new picture. Manual sign out is similar to staff, but they are offered a screen of images to choose from (the one they took on arrival), and should take less than 30 seconds, probably less than 10 if they've used it before. You can pre-fill this for visitors you know will be arriving, and skip them from picking a name on the screen to the picture step (you can skip the picture too but their badge won't have a picture on, which kind of defeats most of the point).
The barcode process: to sign staff in, and sign both staff and visitors out on the console is quick, staff are prompted for the reason they are leaving, and then it displays " signed out" message, probably under 5 seconds for the whole process (getting the barcode to scan is usually the longest part as people usually just push it right up in to the light, but it can only read from about 5 cm, 10 to 30 cm is better).
Staff, and I assume sixthform, can sign in and out on Windows CE powered QuickScan units (looks like a Motorola MK500). It just displays " signed in" or " signed out" for a few seconds (no prompt for a reason for leaving). While is is displaying the message it won't allow the next person to scan. It's just slow enough that a queue can't file past and scan through, but if you are following someone walking past, it's not usually a problem. Visitors cannot sign out this way (I asked the InVentry Support, and they say it is currently impossible to allow this).
Hope that helps.