Eevee way much slower than Cycles I've got a scene where Eevee viewport is so much slower that even makes my whole computer impossible to work with but when I change to Cycles it gets smooth like butter again.
Can anyone tell me what I'm doing wrong?
Thank you
It’s likely that it’s your materials rather than your scene itself that’s causing the lag.
I have had issues in the past where using multiple Noise textures and other texture nodes within a material can really hurt performance on Eevee. This seems to be due to Eevee pre-rendering some surface textures to optimise the actual render - and this seems to cause a performance hit whenever the scene or materials are changed. Try removing your materials one at a time and see which (if any) is causing the lag. You may be able to simplify any materials that are proven to be causing the issue.

Update: I've recently made some more progress on this and tracked down slow Eevee performance to multi-dimensional Texture nodes in particular. For example, using a 3D texture (such as the default for Noise) where perhaps a 2D texture (Noise set to '2D') would do - for example, when using a UV map, the Noise (or other texture) should be set to '2D' to drastically improve performance.
Note that Eevee appears to pre-sample such textures and so reducing the Dimensions to the bare minimum for that case will drastically reduce the overhead (eg, it only needs to pre-sample a plane for a 2D texture instead of a volume for a 3D texture - the extra dimension is wasted processing time if that dimension is never actually going to be used).
Note that in extreme cases I have had Blender crash due to this - when I swapped from Cycles render view (which worked fine) to Material Preview mode (which uses Eevee regardless of your render settings). Reducing the dimensions often resolves the crash - as well as making the interface much more responsive.