Use the Wait Time chart to determine if the Azure SQL Managed instance is experiencing Tempdb contention. Note that Tempdb contention refers to a bottleneck for threads trying to access allocation pages that are in-memory; it has nothing to do with I/O.
The Tempdb Wait Time chart plots wait contention over time for the resource types:
PFS
Page free space
GAM
Global allocation map
SGAM
Shared global allocation map