My game lags after I spawn an object

So in my game, I made it so that if all the mobs die then it spawns a door object (a portal), but for some reason... It lags when the portal has been spawned.

Here's the code I used:

if (!instance_exists(mob_properties)) { instance_create(2050, 230, obj_portal); }

How can I optimize this code to avoid having this problem I'm currently facing?