In recent eclipse (I'm using 2019-06) I'm not sure if it is specific plugins or the platform itself, but on startup it tries to connect to internet systematically, I believe this is to check for updates.
Now in many roaming situations where I don't have network, or in places where a proxy configuration is necessary to see internet (proxy/preferences are configured per workspace, so changing workspace => wait again), eclipse will hang at startup for up to several minutes (feels like two standard TCP timeouts of two minutes).
Is there a flag I can pass at startup, or something I can place in eclipse.ini file that says "forget about connecting"; at least until I click a button that actually requires network (e.g. git push/pull).
So after investigation, the answer at this time seems to be no, there is no such command line flag that would affect the whole platform. Even if you know you have no network, no "start offline" mode is available.
Eclipse does remembers on next restart whether it was online or not last time, in both ~/.eclipse/ and with per workspace .metadata information. But no solution for first startup on an empty account.
Plugin specific tuning might be possible, as suggested in some of the comments (thanks for suggesting that), but customizing plugins one by one is mostly not satisfactory and not maintainable.
In our use case, which was an programming exam on machines for 100+ students with no internet access to avoid cheating, and temporary blank accounts, in the end we had to reconfigure the proxy/network to let Eclipse reach it's update servers (6 of them for a default Eclipse/CDT distro), and then reconfigure them again as we noticed the IP it connects to are unstable, probably due to mirroring of the update servers.
Similar questions yielding this answer: Eclipse hangs intermittently on PC without internet connection
Some people migrating from Visual Studio and complaining about not having this feature : https://social.msdn.microsoft.com/Forums/vstudio/en-US/3532a794-bb99-461e-9e32-a922f5e7a1b8/can-you-start-eclipse-with-an-offline-parameter?forum=tee