Speeds also depend on how many drives are "hooked-up". For example, I have 8 connections, 6 are labeled as 6MB/s. However, if you read the manuals, you will only get "sustained potential of 6MB/s", if you only have 4 units, and all 4 are on the first four 6MB/s connections. (Absolutely NO other drives can be hooked-up to any other port.)
If you hook just ONE drive up to any of the other 6MB/s ports, all ports instantly drop to 3MB/s max. Though, you can hook-up ONE more, to the other 6MB/s port, and they will also still be 6MB/s. Yet, if I have just TWO drives, they can be hooked-up to any of the 6MB/s ports, and run full speed. Adding the third drive must be done with use of the four first positioned ports, or the same as above, with 4+1 drives, will happen. (Essentially, there are 12MB/s per group, in the four ports at the front.)
Here is where it gets buggy...
Add one drive to ANY of the non-6MB/s connections, and the 6 ports turn into 2MB/s. The driver will then "burst", +2MB/s across any of the 6 6MB/s ports, but the four front ports will get an extra +2MB/s, if it is available.
To add insult to injury...
Adding an M.2 into slot-1 will reduce the top-row of 6MB/s into only 2MB/s slots. Adding an M.2 into slot-2 will reduce three, out of four on the bottom row, into 2MB/s slots, but the first one will remain as a 3MB/s slot, with that being the only one which gets +2MB/s bursts, as long as none of the M.2 cards are using ANY transfer bandwidth.
Yes, it is a big mess.
The same happens for PCIe slots. You may have 3 16x slots. But they are not all 16x, if populated. If only using one card, it will be 16x in any of the three slots. But, add one card in any slot, of any speed, and that card now only runs at 8x, and the other two will only run at 4x each, in many instances. (Eg, you only have 16x total lanes for data. Most controllers don't "float lanes". They have fixed reservations for bandwidth by locking-out lanes for other cards, when inserted. Sometimes/Most-times, RAM and DRIVES and INTERNAL VIDEO CHIPS and INTERNAL SOUND CHIPS and INTERNAL NETWORK CARDS, will also use those lanes, unless you disable them. Though disabling them may not actually free-up any lanes, it will reduce the interruptions from any "floating lanes", if some of those lanes are floating, in the 8x and 16x connections.)