The code assist in the Editor window causes a delay. In some cases the delay is related to the display of object. In some cases it is related to the parameter hints option.
This may be due to uncollected stats on a database, causing the query that Toad runs to be slower in some environments.
The size of the list that needs to be displayed for code assist may also factor into the time it takes for focus to return to the editor.
If not being used, certain code assist features can be disabled in View | Options | Editor | Code Assist.
If the issue is mainly with parameter assistance, this can be disabled by unchecking "Enable parameter assistance"
Enable cache for code assist by going to View | Options | Editor | Code Assist, enable "Cache Code Insight results".
If the issue is that too many objects are being pulled back as a pick list, and the list is taking too long to display, the list of objects displayed can be refined. In View | Options | Editor | Code Assist, use the Toad Insight Objects list to filter the types. Check the objects that should be included in the pick list, and uncheck any objects that won't be needed in the pick list.
Tip: Public synonyms can return tens of thousands of results on some databases, significantly decreasing performance. If this object type is unimportant to you, you can clear this object type to improve performance.
Run SQL Tracker/Spool to collect the SQL that Toad sends out when code assist is accessed. Pinpoint that query and test it outside of Toad. Toad can only display as quick as the run time of this query, plus a little more time taken for display on screen. If this takes too long to query, try apply Optimizer Hints in Toad.
Go to View | Options | Oracle | Optimizer Hints. Below the grid displaying views and applied hints, is the Default setting. Use the drop down and select one fo the hints to apply in Toad. Save the setting, open a new editor and test the issue. Do this for each hint in the Default setting to see which applies best to the environment.
Have the DBA/Admin gather stats on the database in question to help with performance.