rapid
Or possibly "steep."
A five letter word that is the opposite of the word "gradual" and contains a second letter of "l" is a popular word game answer. The five letter word being sought is "fleet."
Sudden
The opposite would be sudden or abrupt, so the word could be quick, rapid, or swift.
A five letter word for the opposite of lower is upper.
A five letter word meaning the opposite of hurry would be dally. There may well be others
The opposite would be sudden or abrupt, so the adjective could be hurried or instant.
The opposite of sudden is gradual.
least
night
giant
plain
fresh