Why does the first step "Query Friends for all my friends" take 1.5 seconds? I am struggling to understand this. If this simple table of say 100,000,000 rows is

Why does the first step "Query Friends for all my friends" take 1.5 seconds? I am struggling to understand this. If this simple table of say 100,000,000 rows is indexed on userid and we are only reading and returning say 200 rows for a particular userid what makes it so slow?