DROP TABLE is executed, by default, in Galera Cluster by Total Order replication (TOI) method. The TOI replication will hold total order commit monitor during the whole duration of DROP TABLE execution.
This could be improved by ending the TOI state as soon as DROP TABLE execution releases MDL locks, and leaving the later time consuming file delete operations to happen in background.

This approach requires that all operations happening after MDL lock releasing, must be successful. A potentially offending scenario would be if some other replicated (DML or DDL) statement or transaction would interfere with the DROP TABLE later stage file operations.