Welcome @altavir at bottom of this doc https://github.com/mipt-npm/kmath/blob/master/doc/algebra.md there's note

Fields themselves do not solve the problem of JVM boxing, but it is possible to solve with special contexts like MemorySpec.

What do you mean with jvm boxing?

The problem is actually not specific to Java and JVM. There are a lot of articles about it. For example this one: https://docs.oracle.com/javase/tutorial/java/data/autoboxing.html. In short, if you are using some kind of generic objects in your code (for example, abstract numbers), eventually they be represented in memory like references to heap, not by stack-allocated primitives. It has a strong impact on performance. There are currently no good ways to treat the problem in any language (maybe only in Julia).