The strange lag appears on Kong if you have storage space set to less than Unlimited! Just try to right click, then go to Settings and set there the storage slider to Unlimited.
Try to beat more levels, Poisoned Balls are useless when the enemies become stronger and you still need the defense system then.
The same for Spitting Cactus.
Thank you to all for such useful comments. I slightly updated the balancing system, so bosses and viruses are not so hard now. Upgrades gives more damage rate as well.