WNDR4300 Download Speed

Hi,

If I run a speed test on the internet line coming out of my modem, I am receiving ~400MBPS download speeds.  If I connect it to my router (WNDR4300) and then connect an ethernet line to my PC and run another speed test, I am receiving ~140MBPS download speeds.  I am seeing a similar ~140MBPS over wifi via my phone.  It seems like my router is throttling my download speeds.  What are recommended steps to root cause this issue?

Note: I received this router as a christmas gift from my father about 4 years ago and has been functioning properly over the past 4 years.  I haven't had any issues with it until now.

Thanks,

spgrooms