Does having a longer Ethernet cable slow your connection?

No, it will not slow down a connection but you need to be aware of the maximum length of a copper connection which is 100 Meters. This needs to include the length of your patching cable from the host to the data point and also patch frame to the switch.

However when using CAT6 with a 10Gb interface, you can only use up to 55 Meters and would need to use CAT6A to achieve 100 Meters for this type of transmission.

So if you are going above the specified maximum cable length, you will start to see problems not just relating to speed but also to loss due to the nature of electrical current running through the cable.

The 100 Meters only applies to a single run without any intermediary network device such as switch. If you have a switch in between, you can obviously extend this from port to port which the maximum would apply to for each cable run from device to device.

Using Fibre connectivity, you can extend the range based on what type of fibre and light which is beyond the scope of your question.

For all practical purposes, there will be no effect on the speed of your connection.

There will be a very insignificant amount of delay due to long cables. This wont affect the max speed of your connection, but would cause some latency. pjc50 points out that it's about a nanosecond for every foot of cable length, which is a good rule of thumb used by many engineers when developing systems which are very dependent on latencies on those timescales.

In reality, you will never notice a difference. A "fast" ping time on the internet is 10ms, which is 10,000,000ns. Adding even a few hundred feet of cable isn't going to have a noticeable effect at that point. In fact, nearly every step of the way involves delays which are more extreme than those seen from signal propagation. For example, most consumer grade routers will wait for the last byte of an incoming packet to be received and check it for errors before sending the first byte of the packet on its way. This delay will be on the order of 5,000ns! Given that the maximum length of cable you can run (per the Ethernet spec) is 300ft, the cable length could never cause more than 300ns of delay due to cable!

Sort of, to a very tiny extent.

The longer your cable, the higher latency you experience - gamers call this "ping" time. However, the effect is about one nanosecond per foot of cable, which is unlikely to be noticeable in most cases. Especially as a single ethernet cable is limited to 100m.

This matters for high-frequency trading and occasionally for email.

It doesn't, of itself, affect the throughput or "bandwidth" of your cable.

I believe it can, but not in the way most people are thinking about.

Most are thinking of the extra propagation delay through the cable itself. This is valid, but as people have already pointed out, so small that it's essentially always irrelevant.

There is another possibility though. Ethernet cables come in a few different varieties: cat 5, cat 5e and cat 6 are currently in (reasonably) wide use. Cat 5 doesn't officially support gigabit Ethernet, but with a short (e.g., 1 or 2 meter) cat 5 cable that's in good physical condition, you can often get a seemingly reliable gigabit connection anyway¹.

With a longer cable, however, you could get enough signal deterioration that a gigabit connection was no longer possible. In this case, I believe you'd normally be a 100 megabit connection instead. In this case, you wouldn't just gain some irrelevant amount of latency--rather, you'd have lost a substantial amount of bandwidth.

This wouldn't have any effect on an Internet connection unless you happen to be one of the fortunate few with more than 100 MB/s bandwidth. Access to local resources could be affected much more drastically though.

1. All of these use identical-looking RJ-45 connectors; the difference between cat 5 and cat 5e cable usually isn't obvious except by looking at the printing on the wiring to see which it says.

The standard is 100m (~333.33 ft; 1m = 3 1/3 ft) before attenuation makes the signal unusable, but the direct answer to your question is yes, a long cable can slow your connection. Attenuation is caused by the internal resistance of the copper which humans perceive as lag/slow down of network connectivity. If the cable is under 100m, the slow down is relatively unnoticeable. It can cause issues if you're coming close to that 100m mark though. And keep in mind that the 100m length is measured from the point the cable plugs into the port on your computer to the point it plugs into a device that regenerates the signal, like a switch or a router. (I've personally had to change out a cable to a printer because the ~97m length caused sporadic communication.)

The electrical signal propagation time for a 100m max length ethernet cable is only about half a microsecond. This is far less than the amount of time needed for your router/etc to do their jobs.

This only begins to be relevant when looking at much larger distances: eg From your computer to the server for a game you're playing; however that number is entirely in the hands of your ISP/its partners and the physical locations of you and the server itself.

The electric signal will be slowed down by a minimal amount (afterall it travels almost at light speed), how much time does light take to travel for 100 meters?

timeTaken  = 100/299792458 = 0,00000033 seconds

So it just takes an extra 0,0003 milliseconds which is just 300 CPU cycles (on a 1 Ghz CPU). However the longer the cable the weaker the signal becomes, once the signal is weak enough it will starts to lose bits of information because of interferences, each time a bit is loss, something in the network layer sees that a checksum/parity check fails, and ask for that packet again.

Asking for a new packet will take a very long time.

So as long as signal is strong in the cable, the slowdown would be minimal (it is greater than I expected anyway).

Once you start losing information because cable too long, the slowdown would greatly increase.

Also note that certain communications protocols are timed, so if the cable is too long it may not even be usable because it would go out of sync (that's a by-design issue)