The Motion blur filter for Affinity Photo can be found in the blur menu in the filter menu.
It comes with a number of options to manipulate images as well as presets, The settings available are rotation and radius. The slider can only reach 100px. You can also interactively modify the rotation and radius by moving the cursor over the image, find this much more effective than the panel controls. You have still got to remember to click the apply.
It can be used once or multiple times. You can use the repeat command to apply it over and over with the same settings or go to the blur menu and motion blur and apply with the different settings. You can change the angle as well as the length of the blur. If you set the value to high, the result is basically a streak of colors.
It can be applied to the image on a layer. You can duplicate the current layer and then re-size the layer or rotate it and then apply the effect reducing it to a faint wisp.
It can be applied to the image in a selection and the selections can take many forms such as using selections created by the freehand selection tool or elliptical or rectangular as well as using the selection features such as feather etc.
It can be used in different color channels so go to the view menu and studio and channels and in the panel select the red channel and then apply the motion blur filter and then go to the green channel and apply a different motion blur (length angle etc) and then the same with the blue channel. Return to the RGB channels and you will see a completely different design from that generated by a single RGB application
It can also be applied to all kinds of vector shapes though when you do this an assistant will appear and convert the shape to a pixel layer and the motion blur filter will be applied to that. The vector shapes can take any number of forms such as stars, diamonds, cloud shapes etc as well as vectors that you create using the pen tool etc. You can create many unique designs by combining the effect with other filters such as modifying the shape initially with the powerful equations filter along with mirror filter as well as the distortion deform filter.
It can also be used in combo with the pattern layer from selection where the layer or selection is captured by the command in the layer menu and the pattern layer can then be modified by the effect or you can create the entire pattern and then apply the effect to that. You can then repeat the steps and work the effect on the latest layer.
The effect can be modified by the layer menu and fade command. You can reduce the effect by changing the opacity as well as the blending mode such as darken, lighten, difference and others.
The effects above are all destructive but you can use the non destructive version of this by using the layer menu and new live filter layer and blur and the same effect. As a layer effect, it is applied to the current layer or if there is no layer selected, to all the layers below. The advantage of the non destructive filter is that the speed effect can be modified at any point as well as deleted as well as duplicated. You can also combine it with other live filter layers as well as adjustment layers.
Create a layer and then fill it with a few brush dabs in white and red etc and then apply it to 500px etc and use the interactive feature to angle and strength the lines generated. You can also duplicate that layer multiple times by holding down the alt / option key and creating complex streaks of light for any image.