@Laurens
Looks like those threads were idle at that particular moment. You could repeatedly get stack traces from those threads to see what they are doing when they are not idle.
Since openHAB 2.3.0-SNAPSHOT build #1230 a new Karaf version has been integrated which you might want to try to see if it perhaps also fixes the high CPU due to the features-1-thread-1.
See also: Unit-Support, KNX 2, Karaf 4.1.5 Upgrade and more!