Layover in Lisbon

Clearing customs and then going back through security is something that happens when flying into the US, but it doesn't happen in most foreign airports. In Europe, it generally only happens when one has to transfer between terminals in an airport where those terminals are not connected airside (post-security).

So, in Lisbon, that would only happen if you were transferring from Terminal 1 to Terminal 2 or vice versa. Terminal 2 is only used by low-cost airlines such as Ryanair, EasyJet, Vueling, etc, so if (as I suspect) you are flying through Lisbon on one ticket, you almost certainly won't have to change terminals.

In addition, during these COVID times, Terminal 2 is closed and all traffic is going through Terminal 1. So, it will be a while before this is a concern for anyone again.

I was in LIS in 2016 and thought it was a good airport, though I did have to use a bus gate.