A useful mod with which you can get an analysis of the performance of Minecraft and find out the reasons for the slowdown of both the game client and server. You can find out the available mod commands by entering in the chat / csampler help or / sampler help (for the server).
Sampler Mod for Minecraft 1.12.2
In addition to this, the mod will help you to slightly increase the FPS in the game, since you can analyze the data received and change the game for the better.
Download Link: sampler_1.84_1122