That's why you need some sort of productivity journal.

Show that you were completing X number of tasks per month, and after a couple months show them that your productivity is down 10-20% in terms of tasks completed.

Make it extremely easy to present, and if your Manager agrees to work with you on it, make it look like the work to compile this report was all theirs. In other words make them look good and do their job for them.

My FAVORITE EXAMPLE in the road map for Star Citizen. That game's development is at an agonizingly slow pace, but if you were the Manager who put together their roadmap it would be instant promotion. It's goddamn BEAUTIFUL. And their is no way the team could be fucking up if the Roadmap looks this good.

https://robertsspaceindustries.com/roadmap/board/1-Star-Citi...