I heard when we delete a document it remains on disks and next queries will filter deleted documents, is it true?
Segments are immutables so yes technically they are still on the disk until the next merge.
Yes its true, when you issue this command : "GET /_cat/indices?v " on sense , you will see the doc.deleted increases.
So the search performance decreases? Until next merge?
You should not really notice.
See https://www.elastic.co/blog/lucenes-handling-of-deleted-documents for more details.