Internet Speed Question

Hello,

I was just asked a question related to internet speed and whether or not upgrading cables in their home would impact them any. That is, the cables connecting their devices to their router/modem. Not the local loop connection or ISP.

Embarrassingly, and after having taken 4 CCNA courses in university, I struggled to definitively answer with "no".

I realize internet speed means a different thing to some people, but their main gripe was supposedly not getting their promised gigabit download speeds. Instead, they were getting 89 down and 19 up.

As it turns out, their main focus was a wireless device, which kind of wrapped the issue up.
The impromptu question however, did still kind of bug me.

TL;DR: Since my CCNA courses were always under the pretense of an organizational environment, I was wondering if any changes made to anything connected to the modem would make a difference for speed in a home environment (i.e. cables).

For example, if someone had a CAT 5e cable connected from their desktop to their router/modem (w/ all ports supporting gigbit speeds), and they were only paying for 300 Mbps down, they wouldn't ever see anything above that, correct? Even with the faster cable they own and choose to use to connect to their router/modem, they would only receive the speed allowed by their ISP via the local loop connection.

The other way around too, if they were supposedly getting gigabit speeds but they were only connecting to their router via CAT 5 (not CAT 5e) cable, they wouldn't get above 100 Mbps, correct?

I vaguely remember a lecture where the phrase "the speed is equal to the weakest/slowest link", but I don't remember if that's how it's always applied and if that's the only parameter involved. In my head, that makes sense anyway.

Josey Wales said:

If you bypass your router and plug directly into your ISP's modem, you will get a higher rate. I was not getting the speed that my ISP said I should be getting which was 300 down and 100 up. I called my ISP and the tech told me to bypass the router and my downward speed went to 320. The router does and will slow your speed down, but it is a necessary evil.

I'm assuming that's 'cause it's doing other stuff under the hood (translation, filtering, firewall?).

Also, I just realized that perhaps my question wasn't related to speed at all.

Logically, I'm sure plugging any device directly into the modem would get faster speed but at the cost of security.

Theoretically, however, if I connect my desktop to a switch with a CAT5 cable and that switch connects to a router with a CAT6 cable, would my bandwidth be limited to 100 Mb/s or am I mistaken?

Also, is the bottleneck really just the router or do cables play any part of that?