PCI Express slots rules and speeds that are good to know.
This really becomes important when adding cards that need a fast set of lanes, or x4 lanes to get speeds you need,  for example a fast USB-C 10Gbps card or SATA-3 (6Gb/s)

 I made a photo that tells lots about this DESKTOP slots used on all modern PCs (not old legacy PCI)

Speed is mostly an  issue  with GPU cards,  but SATA3  (or USB-c super  fast ports at
10Gbps ?) lanes do count for sure on old PC's.
In the case of USB-c the chips used will be your limit and many are not up to the task yet.
The new Thunderbolt 3 protocol which can support up to 40 Gbit/s transfer speeds and can even offer dual 4k monitor support over a single cable. (the cards to do that are super expensive if PC lacks them now)
For SATA-3 (6Gb/s) cards speeds only need to be 500MB/s and to attain full speed and using lane counts to do that act.
I do have my HP z600 upgraded to V3 rate card for SSD (2.5"sata drive) and boots from the SSD via card. (buy with care)  V1 can work if x2 lanes cards) V2 needs only 1 lane and is top selling cards.
More speed facts, from the amazing wiki.  a very nicely made table. ( 500MB/s at least, is what most want for SATA 6G cards.) 

Folk saying v1 is too slow are full of beans, buy card with x2 or more lanes !
Note that only version 1 will limit a card below, but if I bought a card with x2 to 4 lanes , then even old V1 PCs can be upgraded to SATA-3.

Here is my Sata card  and SSD 2.5" benchmarked here.
SSD V2 PCI-e x1 lane. (even way faster if a RAID card is used, even  a cheap LSI-9211 RAID card can do that!) Even run 2 SSD as one drive for 2 times the speed. RAID0
SATA -3 is  6Gb/s (Gigabits not Bytes) so 6000 divided by 10 is 600MB/s  raw and no USB goes that fast, ever due to 8/10 encoding (yes in binary raw viewed )
The through-put  is about 480MB/s actual end to end usage.
You can not go faster than the 2.5" SSD drive chips allows, and is why we have M.2 drives that go vastly faster. (with vastly faster Flash chip inside)

The M.2 below screams fast there is no SATA buss to slow it down. Some can read at 7000MB/s  (bytes not bits) M keyed are  x4 lanes.
One can buy a naked PCI-e to M.2  M-key x4 PCB card and then use this NVME below. To make this boot the PC BIOS must allow that.

There are many Gamers (mostly) that can not get their GPU card running at a full X16 slot speed. (drivers are good)  (or is a waste of time trying gee wiz, a GS8400 is slug card circa 2007, does a 5 whole FPS in FURMARK .)
The CPU must have 16 lanes or old PC south bridge have it., and the slot too, the lower slots even with 16x Socket is 8x (SLI sure) 16x slot is most near CPU.
Or the MOBO has other shared X lanes, hobbling the main X16 lanes per below.
NO better answer I have ever seen is on this 1 page,  this guy (
Akshat Verma) NAILS this question 100%, what great work, and complex topic...
Can I add one more,  sure,  if the card is slow any why, the X4 or x8 is plenty fast enough !  Like old 5FPS cards 10 years old at are slugs,  you can not make GPU chip run faster with  added X lanes ever, just like YUGO car on and empty 8 lane freeway, hopeless.

Let me say this,  most (not all) mobo makers do not even publish there MOBO BLOCK diagram showing how this all works. (I yes look for X lane dynamic switches (yeah shared)) like seen on M.2 mobo.
Example of good. (MSI.COM THANK YOU) The sharing is clear here. (mine is Gen7 intel I7) Note how M.2  impacts SATA6 !
The "devil really is in those details".
( vast PCs made and chips sets there is no LAWs on this, what matters is what you have, not others, ok?) ASK you OEM for a real diagram 1/2 refuse, so buy only from friendly makers.
One more ASUS ROG.
Intel did not make your MoBo,  your OEM did, and only that matters. (wrong CPU not present) Z270 bridge south.

version 1.  10-1-2020