The major USB speed improvements are shown on the below timeline.
2019 - USB 3.2 Gen 2x2 (SuperSpeed 20 Gbps) 2,500 MBps
2013 - USB 3.1 Gen 2 = USB 3.2 Gen 2 (SuperSpeed 10 Gbps) 1,250 MBps
2008 - USB 3.0 = USB 3.2 Gen 1 (SuperSpeed 5 Gbps) 625 MBps
2000 - USB 2.0 (HiSpeed) 60 MBps
1996 - USB 1.0 (FullSpeed) 1 MBps
1996 - USB 1.0 (LowSpeed) 0.1 MBps

Fully backwards compatible

Each successive generation is fully backwards compatible with previous versions. Older devices always work in newer sockets and newer devices always work in older sockets, albeit at the slower speed.

Speed increases

USB 3.1 Gen 2 devices can reach speeds of up to 1,250 MBps. USB 3.1 was finalized in 2014, when maximum transfer speeds doubled from 625 MBps to 1,250 (matching Thunderbolt). Maximum power output also increased from 10 to 100 watts and a new reversible C type plug was introduced. These upgrades to the specification were entitled USB 3.1 Gen 2 rather than USB 4.0.

USB 4.0 speed and release date estimates?

There are currently no official proposals for the USB 4.0 specification however if its speed increase is in line with previous spec revisions, it should offer transfer rates of approximately 10,000 MBps, ten times faster than both its predecessor and Thunderbolt. In terms of release date, the previous major USB releases are spaced out by approximately six years so we estimate around the 2020 mark for USB 4.0.

Can USB 3.0 hold the fort?

USB 3.0 is still a reasonably safe bet in today's market because there are still very few devices capable of saturating a USB 3.0 connection. We lab tested 30 of the fastest USB 3.0 flash drives and found that in terms of sequential speeds they operate far below the USB 3.0 limit of 625 MBps. Furthermore, in terms of the generally more important random 4K speeds, Flash Drives are still struggling to break 10 MBps. Bleeding edge Solid State Drives on the other hand are a different story. The latest record-breaking SSD from Samsung, the 840 Pro, has a sequential read speed of 511.17 MBps and would likely be hindered by a USB 3.0 connection because real-world USB 3.0 speeds (400 MBps) are significantly lower than the theoretical 625 MBps limit set by the spec.

How fast are the competing external interfaces?

The below chart illustrates release dates and maximum transfer rates for some of USB 3.0's top competitors.
Thunderbolt (2011) - 1,250 MBps
USB 3.0 SuperSpeed (2008) - 625 MBps
Firewire S3200 (2007) - 390 MBps
eSATA (2009) - 370 MBps
Firewire 800 (2000) - 98 MBps
The majority of the above theoretical figures translate into comparable real world speeds but USB 3.0 is an exception. Real-world maximum transfer rates for USB 3.0 are capped at around 400 MBps.

How does USB speed compare to internal interface speed?

Despite the impressive improvements in the USB spec, its speed still pales in comparison to internal interface transfer rates. Internal interfaces are used to connect components like graphics cards, network cards and disk controllers within a computer. These devices demand far higher connection speeds than the external peripherals that USB is designed to serve.
PCI-E 3.0 x16 (2011) 16,000 MBps
PCI-E 2.0 x16 (2007) 8,000 MBps
PCI-E 1.0 x16 (2003) 4,000 MBps
AGP 8X (2002) 2,100 MBps
USB 3.0 SuperSpeed (2008) - 625 MBps
SATA 3.0 (2009) - 600 MBps
IDE ATA100 (2002) - 100 MBps

Why are speed increases needed?

A few years ago 250 GB was considered enormously large for a hard drive, nowadays 2000-4000 GB is quickly becoming the norm. High definition audio, video and photo files are primarily responsible for these increased capacity requirements. As Apple continues to push the envelope with its retina display, devices are able to display more pixel depth and correspondingly those old jpg's that used to look just fine start to look poor in comparison to higher definition images. The same is true for audio, originally the bitrate for an mp3 music track was typically around 128 KB/s, nowadays lossless formats are increasingly popular and they run at bitrates ranging from 400 KB/s upwards. Finally video file sizes have also grown enormously e.g. blue-ray. Copying a 25 GB blue-ray would have taken approx 15 minutes using USB 2.0, with USB 3.0 it would take just over a minute, with 10 Gbps USB 3.0 it will take less than 30 seconds and with USB 4.0, when its invented, it will no doubt take less than 15 seconds!

Are there any non speed related improvements?

USB devices include fans, keyboards, webcams, mice, sound cards and more. For the majority of these peripherals speed isn't relevant as the specification already allows for much greater bandwidth than most of these devices will ever require. Considerations such as energy consumption, cable length and the ability to supply power are often more important to peripherals and since these factors are improved by the spec upgrades even speed insensitive devices benefit as the USB specification progresses.

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