This is information only. Based on my experience in the computer industry.
Updated: 3/7/2020
Last Update: 2/10/2009
As of 2/2009 the top CPU on market is AMD Phenom II Quad Core 3.0 GHz or Intel i7 3.0 Ghz.
As of 3/2020 the top CPU on market is AMD Ryzen™ 9 3900 12 core 3.8 GHz or Intel i9 8 core 3.1 Ghz. This will be the basis on the information given.

Data Transfer Bottlenecks

Lets get the dictionary out of the way so you know what I'm talking about.
Bottleneck = Part of the data transfer process that slows down the whole transfer process. Like the bottleneck of a coke bottle, flip it upside down and all the liquid is slowed down trying to get through the 'bottleneck'.
Mbps = Megabits per second (Notice the small & big b's)
MBps = Megabytes per second
Gbps = Gigabits per second
GBps = Gigabytes per second
Tbps = Terabits per second
TBps = Terabytes per second
Kbps = Kilobits per second
KBps = Kilobytes per second
Small b is bits, BIG B is bytes
Why the hell is stuff in bits and bytes? Short and sweet, hardware speeds are determined in bits and file sizes are in bytes. Internet, network, etc are done in bits. Hard drive sizes, data transfers and so on are done in bytes.
Their is 8 bits to every byte. Pretty simple calculation.
Since MP3 files can be around 9MB (megabytes), then this would also mean they are 72Mb (megabits). 9x8 = 72
Ever wonder why you couldn't download that 9MB MP3 file in 1 second on your 25Mbps internet that you pay for? Well, this is why. Your 25Mbps download you pay for is always in bits. If you want to know what your max speed is in bytes, then take it and divide by 8. The internet you have actually can do 3.125MBps (25 / 8 = 3.125). So your 9MB MP3 file will actually take roughly 3 seconds to download. This is in a perfect world as always with residential internet speeds.

The question has always been, what is my slowest link today for data transfers. I found out the hard drive was my bottleneck on a gigabit network, so I came up with the benchmark speeds of a typical hard drive. No longer is the network slower, gigabit networks are faster than your regular joe smoe hard drive.

Today, the slowest link I come across is the gigabit (1000 Mbps or 125 MBps) network as most machines now have flash drives that are reading and writing faster than the network and most definitely faster than the internet.

In most average homes, your slowest data transfers will be the internet, if going through the internet or your network, if you're just transferring to another machine in the house. Below I will give basic speeds of certain devices. This will be the common factors, some people can get better speeds, some get worse.

Wireless Speeds:
If it is consistant and/or fast speed you want, then wireless is not the way to go. Wireless is for convience, not a permenant solution. Many factors interfere with wireless slowing it down. Stuff from cell phones, microwaves, and X10 cameras can slow your speeds down. Packet crashing can also slow it down, but this is to technical to write here.
Fastest perfect world speeds:
802.11g = 54 Mbps
802.11n = 600 Mbps
802.11ac = 800 Mbps
802.11ax = 2294 Mbps
More realistic speeds:
802.11g = 23 Mbps (2.88 MBps)
802.11n = 74 Mbps (9.25 MBps)
802.11ac = 300 Mbps (37.5 MBps)
802.11ax = unknown

As you can see, using wireless except to the internet will most likely be your bottleneck.

Ethernet Speeds: (Wired Networks)
Fastest perfect world speeds:
10 Gbps (Cat 5e, Cat 6, or Cat 6a)
1 Gbps (Cat 5+ or Cat 5e or better cabling)
100 Mbps (Cat 5 or better)
10 Mbps (Cat 3 or better)
Data has slow downs as well. Mainly due to header data, checksums, wire resistance & interference that consume the bandwidth, so in the real world the speeds are more like:
9987 Mbps (1248 MBps)
995 Mbps (124 MBps)
97 Mbps (12 MBps)
9 Mbps (1 MBps)

Internet Implementation: Biggest issue on internet I see across the board is disasterious implementation of their wifi for the house. Remember if your wifi sucks your internet will as well. A good bit of people use their internet providers router/wifi/modem which in really bad cases is in the basement. Most are in one end of the house. Ideally you want your wifi to be in the middle of the house. Each wall the signal goes through loses a chunk. All too often people spend tons of money increasing the speed of their internet to 100mb+ to make up for the poor wifi design when they only need 25mbps. I have 25mbps as my kids youtube constantly, 3 tv's on internet streaming, a media server and game server with no speed issues.
Once you tweak the wifi. I then tweak the router and give priority to certain things. Kids get low priority, myself and spouse get medium, server gets high, phone & tv's get ultra high.

Hard/SSD/NVMe Drive Speeds:

Hard drive speeds are based on your normal drives in use today. You could purchase better ones of course, but I didn't have the resources to test all of them.
4200 RPM - 4MB cache - (20 MBps) Read/Write
5400 RPM - 4MB cache - (30 MBps) Read/Write
7200 RPM - 64MB cache - (80 MBps) Read/Write
10000 RPM - 8MB cache - (85 MBps) Read/Write
15000 RPM - 8MB cache - (130 MBps) Read/Write
64GB SSD Samsung - (130MBps)Read (105MBps)Write
256GB SSD Samsung - (560MBps)Read (527MBps)Write
250GB NVMe Samsung - (844MBps)Read (809MBps)Write
Hard drive speeds vary depending on fragmentation and other factors, sometimes you'll get higher than this, sometimes lower. If you defrag your system often or use Linux/Mac, then this will be your average speed. Sadly, people look at size of the drive and don't look to see what it is. I cringe on a machine with a nice 3TB drive but it's a 5400RPM.

USB Speeds:
USB 1.1 = 12 Mbps (1.5 MBps)
USB 2.0 = 480 Mbps (60 MBps)
USB 3.0 = 4.8 Gbps (600 MBps)

So as a summarized version. If you want to max your data transfer speed, then find your bottleneck and fix it. If you do alot of data transfer between computers on a home LAN, then trash your wireless and get wired with gigabit ethernet and get some 7200 or even better SSD/NVMe drives. Get drives with a high cache as well. A 7200 RPM drive with 64MB cache will almost max an ethernet network. Moving between SSD's or NVMe's will require 10G networks.

As far as transfer speeds through the internet, check wifi speeds (between PC's using perf), setting priority traffic. After that and still slow then get a faster internet connection and that costs alot of doe! If you only save and transfer between the internet and your own PC, then you won't need to fix anything because chances are, it is your bottleneck.

Examples of common bottlenecks in a normal users arena:
Q1 - Data from desktop PC (7200RPM-8MBc) to external hard drive (7200RPM-8MBc) using a USB 2.0 connection.
A1 - No bottleneck, USB 2.0 and both hard drives do about 60MBps.

Q2 - Same question above, but external hard drive is an SSD.
A2 - A couple bottlenecks here. The usb 2.0 and desktop hard drive can't keep up with the SSD. HINT: Don't buy a usb 2.0 SSD external, you'll never utilize it to its full potential.

Q3 - Family man downloading mp3's from the internet with his 100Mbps roadrunner and SSD desktop hard drive. Internet modem is connected to a 50Mb switch along with his PC which is a 1 gigabit NIC.
A3 - Lets extract those numbers. 6.25MBps Internet, 500MBps SSD, 12MBps network switch, & 124MBps gigabit card. Remember you will always go the slowest speed in the whole chain! So in this instance, your internet is your bottleneck. This is very common.