MacSensei's Mac
All the sudden, my Mac mini started to respond slower with every click I make. Checked Activity Monitor and a process named "lsd" is capturing 1.1 GB of RAM. It is controlled by root user (Mac OS itself). I dit force-quit but it came back automatically which I thought it would do. Then, the size of memory used started to grow bigger and bigger. So, I shut down my Mac then started up again. The problem remained. So, I figured something happened to this process since it is named "LSD!". I restarted Mac with Shift key being held down until Apple logo appears. It goes into "Safe boot". During Safe Boot, many small files used by Mac OS will be deleted (or refreshed).
Voila! It did the trick, "lcd" is using only 28.9 MB now. Back to normal.
Screen Shot 2019-05-11 at 9.44.30 AM
Mystery solved.

Not quite. It started to grow again next day. Since I use many apps simultaneously, one of apps must be causing the problem. First choice of quitting was Firefox. Restarted my Mac. It seems lsd is staying small.