Why is Internet connection the first thing I should check up?

Network bandwidth is a factor limiting the performance of a distribution system. Most of the information sent is the video stream. Upload bandwidth should be at least 2MBit / s per camera. Consequently, the minimum required connection speed can be calculated by multiplying the number of cameras by 2.
For example, you have 10 cameras that you are going to connect. The channel bandwidth for one camera is 2 Mbps. This means the total bandwidth required is 10 times 2. That is 20 Mbps.
The more you have - the better. You can always check your upload bandwidth on Speedtest.