Network is slowing down in one room when multiple computers connected.

First of all, I am incredibly new to networking, and IT in general (currently studying diploma). Got this job as an Assistant Technician to get experience, but the Technician quit after a week I joined so now I have to do his job as well. I have managed so far, but I can't solve this issue. Basically we have this room, with sixteen computers connected via a switch, to the network. (So, server--> MainSwitch --> RoomSwitch--> PCs). When I connected a couple of PCs (four to be exact), the network ran just fine, but when ever the fifth (and more) PC is connected the whole network slows to a crawl.

With 4 PC: http://ift.tt/2DMxaLN

With > 4 PC: http://ift.tt/2EwyQdr

The switch in use is an Allied Telesis AT-Fs750/24 Websmart switch. http://ift.tt/2DM31Mz

And this is how its connected: http://ift.tt/2EANoJm

Please let me know what else I need to provide for you to help me fix this issue. Thanks for any and all replies to this :)

(PS: Sorry for bad pictures. Took them in a hurry since office was almost closing)