Know what the mod promises
Placebo is a shared-code library. Installing it by itself is not an FPS optimization strategy. A small jar size also does not establish the memory or processing cost of the content mods that call into it during gameplay.
Use the pack's requirements
The project overview does not publish a separate hardware minimum for Placebo. Start with your Minecraft version, loader and modpack author's guidance. Allocate memory through the launcher profile and leave capacity for the operating system and other applications instead of assigning all system RAM.
Measure a repeatable scenario
If performance changes after an update, compare the same world, settings and activity in a backed-up test instance. Note changed mods, render distance and resource packs. Distinguish client frame-rate problems from server tick delays before deciding which component to investigate.