Recent studies using working memory (WM) span tests have reported that processing activities negatively affected memory performance on the one hand and storage activities adversely affected processing efficiency on the other hand. We examined this processing-storage relationship in two WM span tests that required participants to maintain spatial or verbal memory items while engaging in mental letter rotation, focusing on an interference effect derived from this visuospatial processing task. Results showed that the mental letter rotation task interfered with memory of verbal as well as spatial items, indicating the similarity of these two span tests. Furthermore, the two span tests produced a proportional slowdown in processing speed toward the end of each span list. We discussed possible mechanisms in which the processing and maintenance activities disturb each other from the perspectives of domain-specific representation-based interference and domain-general attentional switching in working memory.