Various features in CCleaner need internet access to work properly, particularly in the Pro version.
As well as checking for updates, it checks if you have a valid licence, then there is the driver updater, software updater, Speed and Security in Health Check, all need to check against lists online.
Plus the performance optimiser, the bug reporter, the emergency updater, and various other functions all need internet access to work fully, or in some cases to work at all.
I'm not sure why yours is taking so long to launch if you are not connected though.
Mine currently takes about 6 seconds if not connected, and about 2 if connected.
Although launching speed can vary quite a bit at times.
Personally I put that down to security apps such as your AV checking what CCleaner is trying to do, I've noticed in the past that CCleaner launch can slow, sometimes drastically, following a definition update to Windows Defender, then speed up again following the next definition update.
You could try reinstalling CCleaner using the installer from here and seeing if that fresh install makes a difference to your loading speeds: https://www.ccleaner.com/ccleaner/builds
Don't uninstall first and the new install will pick up your licence key and settings without you having to enter them all again.
PS. Are you using the Winapp2 extension by any chance? That can slow down loading of CCleaner, particularly if you haven't trimmed it with the winaptool.
I'm not using the Winapp2extension. I've reinstalled Ccleaner as you suggested and that did improve loading times, its now 20 seconds when not connected to internet.
As far as it needing to connect to the internet is concerned, I understand the reasons you give, what I don't understand is why it needs to happen before, or while the program is loading.
Another reason it takes so long may be that it doesn't give up trying to phone home soon enough for offline usage, so that involves a wait that's too long. Same happens with Recuva (old version from years ago), no internet connection equals an unusually long wait.
With how allot of software operates nowadays I'm surprised so much of it works at all offline, i.e.; no phone home capability equaling unusable - that's how some video games work with their DRM check/validation.