Making code faster requires insight into the particulars of how computers work. Processor instructions, hardware behavior, data structures, concurrency; it’s a lot of black art. Here’s a few things to read on the forbidden lore of fast programs:
Lockless data structures, the easy way. A Java lock-free data structures deep dive. How do those fancy java concurrent libraries work? Fancy processor instructions! Great deep dive.
Now is an interesting time to be a bottleneck. Your bottleneck is dead. Hardware, particularly IO, is advancing such that bottlenecks in code are exposed. If you’re running on physical hardware, especially if you have solid-state disks, your bottleneck is probably language-bound or CPU-bound code.
Go forth, read a lot, measure twice (beware the red herrings!), and make faster programs!