It tore the elm-tops down for spite (3)
The power of the storm is personified in the opening lines of the poem. It's important to remember, though, that the poem is written from the point of view of a very unstable mind.
When glided in Porphyria: straight She shut the cold out and the storm. (6-7)
Despite the awesome power of the storm, Porphyria seems to be able to "shut [it] out" almost without effort.
[…] she Too weak, for all her heart's endeavour, To set its struggling passion free From pride, […] (20-24)
Love, as we've seen, is figured in terms of freedom and constraint. Porphyria's "weak[ness]" here suggests that she's neither as powerful nor as free as she seemed at the beginning of the poem.