As the screeps world grows ( not with players, but more rooms for the existing ones ) and the server tick times are so dramatically slow ( currently about 5s from a bit over 2 seconds when i stopped playing half an year ago ), here is a suggestion of at least reducing the server load on the invaders and adding a bit of logic to them. 

At the moment the invaders spawn at about 80-120k harvested energy in each room, so my suggestions are as follows:

- spawn invaders based on a time period
- give notification of incoming invaders (let's say 500-1000 ticks before spawning) that if the room is observed (or check in some manner) you can see a "signal" of the incoming attack and then decide how to respond
- increase that time period based on their success/failure  - meaning that If those invaders don't manage to kill even one creep - next time spawn them later, but if they do some damage (break wall/kill a creep) - next time spawn sooner and sooner. 

This will allow the following:

- the players that can successfully deal with them - be less and less invaded thus freeing CPU 
- more natural feel of those invasion, having the option to get alert of their incoming attacks
- more logical invasions focused on the rooms that are not protected enough
- ability to reduce those invasion attacks globally on the server during outages 

and to quote Sergey - Be Happy!