P-channel MOSFET switch - very high Rdson
If your voltage is at least 10V you're guaranteed to have &lt; 140 m\$\Omega\$ Rds(on) with the junction at room temperature (more when hot). 

At Vgs(th) - which could be as high as 4V, you are guaranteed to have at least 250uA flowing so equivalent resistance of 16k\$\Omega\$. 

If your input voltage is appreciably less than 10V you should use a different MOSFET- one that has specified and acceptable Rds(on) at a Vgs that is equal or less than the actual drive voltage. These MOSFETs are called "logic level". 

For example, an AO3401A has less than 60m\$\Omega\$ Rds(on) with a 4.5V drive voltage.