I keep my Mac on and do not restart often, however this slows the processing of nodes. Has anyone else experience this and how to overcome it.

You mean that running your Mac day and night slows down KNIME? Do you also keep KNIME running all the time?
I do the same on Windows and “only” see that KNIME eats RAM if I load several workflows and use them. So I restart KNIME (and close not used workflows) from time to time and get free RAM. But this does not slows the processing (if RAM is not the problem) since that should be only CPU dependent.