Hi,
first thing excuse me for my bad english, it's not my first language.
When I'm attacked by enemy wave came from some portals I usually make a "one way" for them so I could fight them taking my time.
And when there is more of one portals at the same time I rarely finish all the enemy from the portal so when the day is coming the enemy go back to the portal or disappear.
When the disappear point is reach in time the game lag (slow down) for 4 or 5 seconds. And this at every each "end" of "portals waves".
My PC :
Intel i7 4790K
16 Go Ram
GTX 970 4Go
I don't think my configuration is to "weak"

Oh and I'm on the game version v1.2.010 (no beta selected in steam)