How to understand "and" here

Well, a few issues here.

The way I read this, it could be re-written “enabling this behavior while in streaming mode can result in both reduced performance and increased stream state requirements “

But i think you are making a big logical error in concluding what you do in your final sentence. It does not imply at all that the only in streaming mode can performance be reduced.

Enabling this behaviour may reduce performance and increase stream state requirements in streaming mode.

The 'natural' way to read this as an English statement is:

Enabling this behaviour may

(a) reduce performance and

(b) increase stream state requirements in streaming mode.

That also appears to make sense as a technical explanation. Performance is reduced in 'working' mode and state requirements are increased in 'streaming' mode.

However, without learning about this software in detail, it is impossible to say whether that is what is intended - the grammar alone is not sufficient. Very often the only way to distinguish meaning in ordinary English is from understanding the context. This example is written in ordinary English with a couple of technical terms thrown in.

My advice is keep reading. Once you understand everything else, this will make sense. If it does not then ask on a specialist forum, not an English-language one.