You started your network with small amount of computers and networking devices, sooner or later you want to add this and that, next thing you know your network got slower.
If you have this condition in your network, you might want to consider reviewing back you what devices you have in your network. You might need to segment your network.
You can use switches and bridges to segment your network, if you use them you separate your network collision domain. Remember what collision domain is when I posted about switches.
Like the above picture, if you use switch to connect 4 computers, then you'd have 4 collision domains. On the other hand, if you use hub, then you'd have 1 big collision domain, this will not only slow your network but also pose security problems.
The above picture looks good, but they're still count as one broadcast domain. To break up broadcast domain you can use routers.
What broadcast means is that computers need to send packet all over the network. One example is when you use DHCP to give IP address to all computers, first the computers will send out DHCP Discover message everywhere saying, help I don't know my IP, can anyone give me one?
The fact that computers send a lot of broadcast when connected to a network, the more computers/broadcasts you have, the slower your network will be.