No it's not. I tried another open source tool, ActivityWatch, first which was heavy. This is a very minimal yet useful enough. Only been using it for 2 days, and tracking myself between work and non-work activities.
ManicTime is more powerful but it's a bit expensive for me.
Quite the opposite of heavy. Currently using 12.6 MB of memory in Task Manager. I've had it running for almost 15 years (I'm the developer of it), usually tracking many hours of activity each day. It stores all information in an in-memory database that uses a very compact memory format and can be set to automatically prune events that accumulate very little time into the parent activity.