This might be classic micro-optimizing, but maybe it's still worth pointing out.
Inside KisDabCache::fetchDab() a lot of KoColor construction and destruction takes place, which is relatively expensive due to the color space lookups and memory allocations in KoColor::KoColor; ok, this is still only about 0.3% of the runtime (see attached profiling screenshot). This patch gets rid of all of the KoColor constructions but one.