To stress what Monty said, anyone can perform a quick test with the Halifax Mk.III and it's 1350x4lb incendiary bomblets in "salvo" version.
On my system I get a short stutter when I drop the load and the game comes to a slideshow on bomb impact, more so when I look at it.
Do this with a flight of four and your game will go bazinga.
As a matter of fact, IL-2 performs bad when a huge number of actors is in view at the same time (here each bomblet is an individual actor, more than 1000 of them at the same time within visual distance is a tad too much) and even more, IL-2 performs quite bad on effects in general.
You can put lots of stress on the game engine by either having extremely sophisticated, huge massive effects (like nuke explosions of some of the fancy effects mods floating around, where certain modders refuse unwaveringly to respect the game limits), by having an enormous amount of effects within visible distance at the same time (like when these 1350 bomblets hit the ground) or by doing both at the same time.
If a mission builder designs his mission so that some 500 bombs or more drop so quick after each other that they're all "active" at the same time, it's no wonder that the game starts to stutter.
That indeed would have to be fixed on mission side.
Best regards - Mike