Like higher dislocation density, it becomes a barrier to crack growth or becomes easy for it.
There is a complexity to this that is associated with how to view dislocations. Remember there are three types of dislocations: 1.) Edge, 2.) Screw, 3.) Mixed. The mechanism of dislocation motion is based highly on the crystal lattice and the electronic interaction of the atom.
The view of dislocations is a bit distorted as well. Remember they occur at the atomic scale while crack propagation is a microscopic phenomenon, though the initiation is always by the creation of two new 'surfaces' at the fracture region. Dislocations will only be useful in inhibiting crack growth if there is pile up and possibly more beneficial when it occurs at the grain boundaries or a node (which is the triple junction of intersecting boundaries of three grains).
This is based on the assumption that the material is metallic.
Figure 11 Tensile strength and Charpy V impact toughness vs. low angle misorientations in martensite
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