Hi @GDeveloper92 Thank you for posting your query on Microsoft Q&A.
My understanding is that you are seeing slow running queries after transferring on-premises databases into Azure managed instance. Please let me know if that is not the case.
In addition to the above answers. You will need to check what limits (CPU, memory, IOPS, etc) are being hit. The portal and/or sys.dm_db_resource_stats have lots of detail on that; if they were running query store on-prem, then comparing plans has lots of info on waits before and after. what's changed - compare the on-prem configuration with the SQL MI configuration. The transaction log throughput limit can be a key difference, since that's not present on-prem.