Frame rate drops when modifying a camera's viewport rect

Hello, about 2 years ago I made a split-screen cutscene using an animation modifying two camera's rect with no issue. However, when recreating the same effect today with Unity 2021.3.2f1 and HDRP 12.1.6, the frame rate drops to a third or a quarter of its initial value ! More precisely, it went from nearly 200 to 40 in my current project, and from 700 to 400 in a blank 3D project. Am I the only one who experienced this issue ?

 

Just downgraded my 3D project to 2020.3.3f1, the highest fps is lower but so is the drop for sure, it goes from 400 to 340. So this issue existed before but must have gotten worse with newer versions of Unity.