We study dynamic algorithms for the problem of maximizing a monotone submodular function over a stream of $n$ insertions and deletions. We show that any algorithm that maintains a $(0.5+ε)$-approximate solution under a cardinality constraint, for any constant $ε>0$, must have an amortized query complexity that is $\mathit{polynomial}$ in $n$. Moreover, a linear amortized query complexity is needed in order to maintain a $0.584$-approximate solution. This is in sharp contrast with recent dynamic algorithms of [LMNF+20, Mon20] that achieve $(0.5-ε)$-approximation with a $\mathsf{poly}\log(n)$ amortized query complexity. On the positive side, when the stream is insertion-only, we present efficient algorithms for the problem under a cardinality constraint and under a matroid constraint with approximation guarantee $1-1/e-ε$ and amortized query complexities $\smash{O(\log (k/ε)/ε^2)}$ and $\smash{k^{\tilde{O}(1/ε^2)}\log n}$, respectively, where $k$ denotes the cardinality parameter or the rank of the matroid.