Improving wifi in our bedroom, currently quite bad. How to find the cause?

I have quite some camera's connected to wifi, over a openwrt wireless mesh network consisting out of 3 devices. I thought our bedroom would perhaps have bad reception, so i installed a new mesh device today to bridge the gap, but speeds have not improved, the two camera's installed there still have a really bad laggy reception. The internet is not the problem> the devices are cut off from the internet by openwrt settings and connected to a LAN recording/live view system, so the lan network is somewhere not working well. the two mesh points show 3/4 bars of range, and other camera's around the house and garden are working without lag. but these two in the bedroom are always lagging, pretty much. I already tried restarting the cameras to no luck.

For the mesh network, i am using 802.11s over 5ghz. no option to use cables.

Is there a way to find out if im using the maximum wireless lan bandwidth already with the cameras or is there another way to debug this?

Thanks!