Whilst a truncate table operation is underway a lock mutex around the table is created to ensure data consistency; if using innodb file per table the physical .ibd file must be removed which is a painfully slow operation under ext3.
However to speed this up a quick solution is to hardlink the tablespace file somewhere else. This will make the removal of the file that MySQL performs a unlinking operation and not an operation involving foreground random IO.
Eg
host01:/databases/data/test# ln Session.ibd Session.ibd-remove
hostt01:/databases/data/test# mysql test
mysql> drop table Session;
Query OK, 0 rows affected (0.07 sec)
Then at some point where there are spare IO cycles
host01:/databases/data/test# rm Session.ibd-remove
host01:/databases/data/test#
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