I don't see how that could possibly be relevant in the slightest.
Spawnkilling is usually the result of using a 'class selector' through events, which, may be hard to do in 2.5 seconds, meaning as soon as they choose their class and whatnot, they are able to be killed. The reason you don't see spawnkilling in servers that don't involve 'class selection' and tiny bases is because those servers allow the player to fight back for 2.5 seconds while taking zero damage, and not get bombed to hell from the outside.
In fact, my server had huge bases and no class selection, and there was not one complaint about spawn killing, even with just 1 entrance and on top of that, the objective was IN the enemy's base, and the enemy players spawn around this objective.
The second point just means that if they are getting spawn killed, it will not last forever, in fact, getting spawn killed will make it end quicker unless it's time based.