。
She's slow at running.
Literal
She [topic-は] running [nominalizer-の] [subject-が] is-slow.
~のが+[adjective] is the nominalize-and-rate construction: take a verb, nominalize with の, and rate it as the subject of an evaluative adjective. 走るのが遅い ('slow at running'), 歩くのが速い ('fast at walking'), 食べるのが下手 ('bad at eating'). 遅い covers 'slow' for both speed and timing — context disambiguates here in favor of speed.