Personally, when the server becomes really laggy. I sometimes wonder if it is my own laptop. I use this software called 'Bleachbit', it is free and I run the program. My dad works in IT and uses it in his work all the time, and I'm going to say it always does the job. It cleans up all my unwanted files and makes Minecraft very smooth to run and I always have a higher frame per second later. This always does the job for me, hope it possibly works for you. However sometimes I do forget to change from 1.15 to 1.13.2 xD. I hope this helps.