m.2 2.0x4 slot question

[DemonGoD]

How much performance drop would you see using a 3.0x4 ssd in a 2.0x4 slot.  My friend has a motherboard with a second m.2 slot on it and wants to add a ssd into that slot.  If the 3.0 SSD is in a 2.0 slot as long as the speed is below the 2.0x4 max will it run at full speed? IE 3.0x4 ssd rated to read 1500 would see no performance drop and one rated at 3000 would get capped. Do i understand this correctly? also should i match theoretical max of 2000 or real max of 1500ish?  essentially he wants the fastest m.2 in slot number 2 that he can use and i want to know which to suggest to him.  How i think it works > its backwards compatible adn the bus just caps it.  I should be looking for 3.0 ssds taht are rated 2000 or below since anything above that will not work at rated speeds. What im not sure on should i really be looking at 1600 and below? 

[Mira Yurizaki]

Unless you have use cases that actually use the bandwidth, you won't notice a drop in performance. For consumer use, the response time of an SSD is its key asset, not its bandwidth.