solved.  I replaced the card with one from another 3847. same DP/N.  physically look good for both.  after replacing it speed went up to closer to 20.  this is the first time I have seen a wi-fi card deteriorate in function.  PS leads were connected tightly to the original card, I do not see any detachment of antenna on front panel.  so i was not expecting a change when i replace the card.  guess I am surprised.  I thought it might be some software issue although the OS was clean installed recently.
Stock Wi-Fi has very slow download speed 1-2 Mbps. Stock wireless using Realtek driver installed by Windows 10 automatically. Added Tplink usb wireless speed back to 20-30How to diagnose or fix the stock Wi-Fi ? Windows 10 clean install already done, no Dell driver I have an older Inspiron 660 which had much better stock Wi-Fi speed also using Windows 10 driver.
Inspiron 3847, slow Wi-Fi