There is an Achive option you can purchase for the Migration Tool Kit.  This will allow you to archive Customer Orders, Purchase Orders, Shop Orders, General Ledger, and Inventory Transaction History.  This will greatly improve your times.

I found that work orders were the cause of many of the performance problems we experienced when we did a similar upgrade.  I wrote some custom code to archive a large chunk of work orders in the historical work order area.  R&D did not like my script, but I shaved half the upgrade time off.  I am sorry I have not been able to find that set of scripts.

Another thing to look at is how many of the post scripts you really need to run.  The ones that do not apply to you do not run those. 

You might have several patches to apply afterword.  There is a feature where by you can combine some of these patches into one install.  This saves the ifs service up and down time between patches.

The work order tasks area when we did ours with the Beta version did not have much in the way of indexes on the JT_Task tables.  I remember adding two for the upgrade and several afterword.

I think these are the three indexes we added, but you will have to test to see if they help you or not. 

create index VAL_JT_TASK_ACCOUNTING_IX02 on JT_TASK_ACCOUNTING_TAB (ACCOUNTING_YEAR, ACCOUNTING_PERIOD)
  tablespace IFSAPP_INDEX
  pctfree 10
  initrans 2
  maxtrans 255
  storage
  (
    initial 64K
    next 1M
    minextents 1
    maxextents unlimited
  );
create index VAL_JT_TASK_ACCOUNTING_IX1 on JT_TASK_ACCOUNTING_TAB (ACCOUNTING_ID, SEQ)
  tablespace IFSAPP_INDEX
  pctfree 10
  initrans 2
  maxtrans 255
  storage
  (
    initial 64K
    next 1M
    minextents 1
    maxextents unlimited
  );
create index VAL_JT_TASK_COST_LINE_IX1 on JT_TASK_COST_LINE_TAB (TASK_TRANSACTION_ID)
  tablespace IFSAPP_INDEX
  pctfree 10
  initrans 2
  maxtrans 255
  storage
  (
    initial 64K
    next 1M
    minextents 1
    maxextents unlimited
  );

I believe this upgrade also pushes you to unicode or double byte words if your database is not already using double byte words.  Do as much of that work ahead of time in your database before the upgrade.  This also means you should check how much RAM you are using.  We found that we had to increase the amount of RAM by about 60%.