How comparable is performance of Kotlin sequences vs Java streams? I'd like to prefer one or the other for consistency, and I usually go with sequences, as it looks more similar to the non-lazy API, good for single operations, and I find it's more flexible than streams. but is there a downside to using sequences over streams?

They are very similar. I'm not aware of any big downsides of Sequences compared to Streams

Kotlin sequences do not have parallel operation option. Although you can still do it with co-routines, it'll be more verbose than Java Streams. No idea about performance, I haven't tested.