Newer versions of SketchUp (16+) have to read each extension’s hash file and decide whether each one of each extension’s code files are in distributed condition. (It’s the signature security feature.) So this slows down loading more than pre-2016 editions.
But, I see you notice this before adding any 3rd party extensions. There are 4 or 5 that come with SketchUp and you could switch off those you do not use.
Another thing is that since about the same time ~SU2016 the licenses became cloud-based. So it might be that something (firewall, AV, etc.) is interfering with the 2 ports that the license check uses.
From the Help Center …
For you techie types, when you authorize SketchUp Pro, SketchUp communicates with the authorization server via the standard ports 80 and 443.
… and …
For your SketchUp Pro network license to work, ports 5053 and 50530 must be open.
…
Was that machine running Win 7 or 8 ?
SketchUp 2018 has a fix for some performance lag especially concerning the mouse …