How about a single map to a structure that contains a bitmap for days visited, and a bloom filter for pages visited. Very low memory and compute requirements.

How about a single map to a structure that contains a bitmap for days visited, and a bloom filter for pages visited. Very low memory and compute requirements. Map<CustomerId, Metadata> Where Metadata is a dateVisited bitset plus a bloom filter (32/64 bits depends on how accurate you need it to be).