"Layer 2" stuff - like Bitcoin Lightning, Ethereum Plasma etc - is an absolutely huge security vulnerability.

Imagine it like this: suppose you have loads of transactions to make with somebody, but the bank can't handle that many transactions. The bank says, you two go off and do all your transactions between yourselves, then come back and report the results to us.

That's what Layer 2 stuff does: instead of committing every transaction to the blockchain, it lets users perform transactions that *are not recorded on the blockchain at all*, and are then eventually added later as a big batch of transactions all in one go. The exploits you could perform with this should be very obvious: you can screw over the other party, claim it was all fine when you commit it to the blockchain, and by the time anyone is the wiser you've already cashed out. The blockchain doesn't know about any of these transactions until one side decides to commit them (and if they're always committing to stay safe, then the layer 2 provides zero extra speed and is useless). Thus: it can't provide extra speed without sacrificing security.

If you are familiar with version control in software, it's a bit like letting users randomly branch off their own version of the blockchain for a while, then commit it back into the main repository later - but there's no conflict resolution. Take somebody's money, spend it, and by the time there's a conflict (because the other person reports a different set of transactions to the blockchain that you did), you've already cashed it out and it's too late to do anything.