A smaller pump makes associated hardware cheaper. For instance, a smaller diameter shaft means a lower cost mechanical seal and lower cost bearings. Cavitation could become a problem as the increase in speed means an increase in the N.P.S.H. required. If you are pumping an abrasive fluid, abrasive wear and erosion will increase with increasing speed. Many single mechanical seals have problems passing fugitive emission standards at the higher pump speeds.
High heat is a major cause of bearing failure.
The higher pump speeds contribute to the problem. The following diagram illustrates the relationship between specific speed and pump efficiency. In general, the efficiency increases as Ns increases. Specific speed also relates to the shape of the individual pump curve as it describes head, capacity, power consumption and efficiency. At low specific speed, power consumption is lowest at shut off and rises as flow increases.