FPS drops with many fireworks with the Trail effect

Not sure if too much can be done about that, fireworks just produce quite a bunch of particles. Your hardware needs to be able to handle that many particles.

OK

As an addition to that, you could also check if your particle settings are optimal to your hardware capabilities.

That's 2^14 particles (I guess it's maxed out and capped to this value). Performance drops are to be expected with that many particles. Especially with a weak CPU like this one.