LL(1) and LR parsers are linear time parsers. ie, $O(n)$
I is True.
PS: Recursive descent parser runs in $O(2^n)$ time.
Recursion needs stack
II is False
L-attributed definitions have both inherited and synthesized attributes. Can be evaluated with top-down-left-to-right style.
S-attributed definitions have only synthesized attributes. Can be evaluated with bottom-up style.
So, in case a definition only has synthesized attributes, we can call it both L and S attributed. Such an L-attributed definition can be evaluated with bottom-up style.
So, III is False.
Code improvement can obviously be done at the user-level by not writing stupid code. And compiler also improves (optimises) the code. Hence,
IV is True.