Does your computer get slow or does your internet access get slow?
I have not used the "App" or know anything about MS Store Apps. If you mean the actual program, then the first thing you should do is go set a bandwidth limit in the settings. Steam can saturate your network making internet access on that computer (or others on the same link) to be very slow.
my internet works fine, but my computer acts vary strange and slow as soon as I install the steam app for my windows 10 PC. and restart my computer.
But, Steam does show up as a vary large startup process by default under the task setting in my windows PC. It takes forever to start up windows. once I install Steam. I can disable the Steam's startup task, in the windows "msconfig"/ System Configuration panel.
but I found out that it's still acts like some type of hidden malware is present on the system, or some kind of severe error once the steam app is installed on a fresh clean operating system.
I tested this theory on 3 different fresh operating systems and I never have any problem until I install steam. I pinpointed all possibility and combined with my knowledge of computers.
I mean nothing seems to happen until I install the steam app for windows 10.
This steam app on my end acts like a Trojan Horse in which is a program that anti- malware programs sometimes don't see as a threat but they cause harmful results and sometimes can go undetected. hence the name.
I got nothing against people who use steam, but I figured I'd tell you what I've experiences on my end for 3 years now