While using Esenthel in the last few weeks I noticed that the performance in the world editor slowly drops (until at ~7fps). After restarting the editor everything is fine for around 10 minutes.

I noticed this behavior in 1.0 and 2.0.

My graphics card is a AMD Radeon HD 7950 with latest drivers installed.

Please verify that your DirectX/GPU Drivers is not set to some debug mode which records performance (this can be system monitoring tools).
As I haven't noticed that behavior on my side.
If other people noticed something similar please let me know.

I have, in editors and in app, framerate drops and goes up after couple of seconds looking in the same direction . With the lowest lod same thing. Using various graphics cards and systems.

I just noticed that this also happens on my laptop with EE 1.0, which uses a GeForce GT 630M.
But this only happens when using the DX9 version of the editor. However, when using DX10 32 bit version I often get this error (there is no DX10 64 bit version of 1.0 :( ).