“But why another object cache drop-in?” you might ask. Because it’s faster than the others, of course.
Under the hood, WP LCache uses LCache, a library that applies the tiered caching model of multi-core processors (with local L1 and central L2 caches) to web applications. In this particular configuration, APCu is the L1 cache and the database is the L2 cache.
APCu is the fastest persistent cache backend you can use, because it exists in PHP memory. However, APCu traditionally can’t be used on multiple web nodes because each node represents a different cache pool. Because WP LCache has a database-based L2 cache, a cache update or delete on one node is then automatically synchronized to all other nodes.