@Gonzo welcome to the KNIME forum.
You could try and give KNIME 10-12 GB but not more and not run any other jobs. Is this a Windows machine and does it have an SSD hard drive.
Then you could explore this collection about KNIME and performance
You might try:
- Garbage collection
- columnar table backend
- splitting the workflow into several parts and calling them separately while resetting them at every start (maybe a last resort)