I always disable any of the core components that I won't use and this is a safe approach. I wouldn't recommend to uninstall at this phase until Joomla removes them from the core as it happened with WebLinks.
For a component being part of the core means that they will be back with the next Joomla update - plus if they are not present, there is some possibility for future issues mostly when updating with database/updating/installation/ inconsistencies.
The padlock means the extension is protected - so you can't uninstall or disable it because it is somewhat required at some level. The unprotected ones can be uninstalled - but as suggested above it's safer to simply disable them.
If with Lightweight you mean to have a minimal admin menu for your admins, then ACL and custom admin menus is the way to go.
And a couple of extra tips I usually do (if the following features aren't needed):
- Disable versions (both the versions component and the setting in articles -com_content).
- Disable Search and Finder and their plugins
- Disable modules
- Disable redirect plugin
- Disable other content and system plugins
Brief explanations on why I disable the above. First this is not done with speed optimization in mind. It is mostly that I prefer to see and have enabled only what I need/use. It gives me a sense of a lighter site, plus it looks friendlier for my admin users. Whenever I need to enable something, it's easy to do it on purpose.
Versions : If left enabled and never meant to keep history of articles changes, I may end up with a huge versions database table. It's easy to truncate it, but in terms of keep things light and efficient I would prefer to not store data that I won't use. Same with redirect plugin, which can easily fill up the database with all sort of 404 urls. Regarding Search plugins, I disable them all if I dont use search, or allow only the ones that I really want to search for (which in most times is content). I don't like when in search results pages it gives me options to choose among categories, tags, contacts etc. Finder, although it needs manually to start making the indexes, if I don't use the smart search I also disable all of its plugins. I also like to disable admin and site modules that I don;t or don't plan to use. This also gives me a shorter list of modules to select when I want to create a new module. Other plugins, I am going one by one in the plugins list and disable any non-needed/used. In some cases (but I haven't tested) this could have small performance benefits.