Hello Héctor Fernández,
It is possible that the backup process is taking longer than expected due to connections being made to the database while the backup is running. This can cause the backup process to slow down
My understanding is Analysis Services doesn’t have an exclusive mode like SQL Server. To minimize the concurrency activity, consider pausing any scheduled data refreshes, queries, or other activities that might interfere with the backup process.
Please monitor the AAS metrics during the backup process and see what is causing the issue. See the below document for more details. https://zcusa.951200.xyz/en-us/azure/analysis-services/monitor-analysis-services
I hope this helps.