Internet speed is the rate at which data is transferred, while bandwidth is the maximum amount of data that can be sent over the internet at any given time:
Internet speed
The speed at which data is transferred between a user's device and the internet. It's measured in megabits per second (Mbps) or gigabits per second (Gbps).
Bandwidth
The maximum amount of data that can be transmitted over an internet connection in a given amount of time. It's measured in gigahertz (GHz) or bits per second (Bps).
Bandwidth is like the width of a highway, while speed is like the speed limit. The wider the highway, the more traffic it can handle. Similarly, a wider bandwidth means more data can be transmitted at once.
Which affect the most please? - Both will effect the speed. Example, for HD video, gaming, etc you will need a higher bandwidth.
Other than on a leased line, most internet connections are shared between users. Hence the reduction of speed during peak hours.
https://lightyear.ai/blogs/internet-speed-vs-bandwidth