User runs a select statement and first set of rows returns quickly. When user moves the scroll bar to the bottom, Toad takes a long time to return the data and seems like it is hanging.
When you scroll down to the bottom, Toad will do a fetch on all the rows for the query. If the table contains many (millions) rows then it will take a while for it to return depending on speed of database, type of data, how many columns, etc.
Scroll only part way down the scroll bar to fetch the next set of data. Fetch size can be adjusted in the Toad options menu in View | Toad Options | Schema Browser | Data and Grids. Set the fetch size scale, which 1 is equal to 500 rows.
Another option to consider is to decrease the OCI Array Buffer size found in View | Toad Options | Oracle | General | OCI Array Buffer size. Default value is 500. Decrease the value to limit the number of rows return during each trip through the network.
If the query is still slow, you may need to optimize your query or tune the database to improve performance.