@Yadz 

Sadly the short answer to your questions is WiFi is slower, always will be.    Let me elaborate a bit

The Router you have is an AX grade WiFi, more commonly known as WiFi6.   WiFi6 can handle speeds around 1200mbps if the client connected supports it, by which i mean it has a 2x2 support (2x radio chips for each frequency).   If your card is 1x1 (single radio) it will be half that so 600mpbs.     So bear in mind that these are IDEAL speeds, you wont get that over airspace, usually its around 50-60% in most cases, so expect a 1x1 card to get around 450mbps.. that sounds plausable to what you are seeing on the router.

OK so the Deco.. its an older WiFi standard (AC also known as WiFi5), this is 866mbps for a 2x2, so 433 for a 1x1.  Again thats perfect scenario, take a 3rd at least.. one would expect a 1x1 radio on WiFi5 to be around 260mbps which is where you are falling.

This is all speculation, you would need to see what your connected device actually has in terms of a WiFi card.

If its a 1x1 card.. in a phone not a lot you can do really apart from updating the Deco to the Deco X range.  

If its a laptop or PC, you could swop the card for a 2x2 or 3x3 and that should beef up your speeds.

From what you are describing.. its smells of a client card limitation!