This research explores if poor grammaticality judgments of late (age of arrival greater than or equal to 12) second language learners often attributed to being beyond the critical period for language acquisition can be better explained by processing difficulties due to (1) low L2 working memory capacity, (2) poor L2 decoding, and/or (3) inadequate L2 processing speed. In Experiment 1, late L2 learners and native speakers performed measures of English working memory, decoding, and speed, and a grammaticality judgment task. Compared to native speakers, late L2 learners were poorer on all measures. L2 span, L2 decoding ability, and arrival age correlated with L2 judgment accuracy. Late L2 learners had less difficult judging some structures (e.g., word order, questions) than others (e.g., articles, regular past). To simulate these deficits, Experiment 2 placed native speakers under stressors relevant to memory (low or high digit load), decoding (noise), or speed (response deadline, compressed speech) during grammaticality judgment. Span and decoding scores correlated with judgment under relevant stressors. The stressors caused native speakers to show selective deficits in judgment, with some structures (e.g., word order) being less vulnerable to stress than others (e.g., regular past). Performance of natives under noise or memory stress paralleled that of late L2 learners, indicating a role for decoding and memory abilities in explaining poor grammaticality judgment.