Before you buy a WiFi adapter. Try to give high priority to the PC in the app of Deco M5 and try different configurations in QoS. The problem is likely to be solved.

Your old wifi adapter might be of the 54Mbps kind...I've got one of those lying round somewhere and they do limit the speed. I think most any wifi adapter you would buy in a store these days would be an improvement!! I'm (definitely) no networking expert but I think 802.11ac would currently be the best affordable ones? I'm in Australia and this is an example from a local retailer here: https://www.officeworks.com.au/shop/officeworks/p/d-link-ac1200-wireless-dual-band-usb-adaptor-black-dldwa182

I personally prefer the shorter stubbier ones compared to the long one I've linked to but maybe longer = better as maybe more space for antenna inside it and being a desktop you could put it at the back or on a USB extension lead.