During ultrafiltration in the nephrons, the diameter of the afferent (incoming) arteriole will be greater than that of the efferent (outgoing) arteriole. This increases the glomerulus blood pressure and helps to force the components of the blood get out of the glomerular capillaries.
If the diameter of the afferent renal arteriole is decreased and that of efferent renal arteriole is increased, the process of ultrafiltration gets hampered and will be slower. The actual pressure gradient will not be observed, and the filtration of all the components will not be possible.