Hi Nicolas
Has this issue just started since you started using 4.1 SP3.5? If so what was your previous version please?
What browser/version are you using? I'm assuming IE? If you are able, can you temporarily try the latest Chrome or Firefox browser?
Also how many Dimensions and Hierarchies are there in your cube?
If this is a large number (hundreds) you may benefit from changing a flag in the mdas.properties file. Open the Analysis OLAP Administration Guide and search "mdas.properties" for details. In that file you want to set the property:
multidimensional.services.preload.metadata=false
Restart the APS after making this change and see if things improve.
Kind Regards