Probably you have something like: [isp_router]---cable---[deco1]- -wifi- -[deco2]

With a desktop cabled to the deco1, all the traffic to the isp_router is cabled, so, very fast.
On the desktop that is cabled to deco2, the deco2 still has to speak to the deco1 thru wifi, and thus will be slower.

The only way to improve the wifi bewteen the decos is to get them nearer or in a location where they can "see" themselves (minimize walls in betwen, etc., the dispoisition of the decos is very important to minimize latency and improve speed between themselves) or, when possible, move the clients to wifi (or use switches, more complicated) and cable the decos between themselves: [isp_router]---cable---[deco1]---cable---[deco2]