You might, for example, be using Linq in your object pooling code to check if there are any available bullets. I was doing that, removing that linq code sped it up about 3x
It is SUCH a shame that LINQ is so slow >_<
Especially if you then compare the performance of LINQ with what Rust offers and you see that in Rust using map/reduce is almost always as fast as a normal foreach loop and often faster than using a normal for loop to loop over your structure. And then there are even moments where it wins from both unless you try VERY hard.