As an interpreted language, some have lambasted APL as unreasonably slow compared to other languages. However, much work has been done to ensure that interpreted code from a typical user is reasonably performant. Since its first implementation, work has been done on compiling APL.
For the user, there are a few strategies to consider for reasonable performance.
Internally, APL arrays are stored as two lists in memory. The first is a list of the shape (although some implementations also include the "stride"). The second is the ravel of elements in the array. Nested arrays consist of pointers to arrays which may be distributed across memory, their use can lead to very inefficient memory read patterns - in contrast to flat arrays which are stored as a contiguous block. This idea is also explored in the study of inverted tables.
For now enjoy a list of references.
- Roger Hui Inverted Tables (Dyalog '18 User Meeting)