Are SSD drives with PCIe 3.0 / 4.0 / 5.0 compatible with each other?

Cross compatibility

PCI Express versions are fundamentally backward compatible. PCI Express 3.0 is also referred to as "Gen3", 4.0 as "Gen4", and so on. A Gen4 or Gen5 SSD can be used in a Gen3 slot, and vice versa.

Performance comparison: example

When using a Gen4 SSD in a Gen3 slot, the maximum read/write speed of the advertised Gen4 SSD will adjust to the limitations of the Gen3 slot. Test runs with a 2 TB FireCuda 520 (a Gen4 SSD) in an older laptop with a Gen3 slot yielded the following results:

These values are at the upper limit of what is possible with a Gen3 slot. Similarly, if you install a Gen5 SSD in a Gen4 slot, the Gen4 slot will be fully utilized, even though the Gen5 SSD will not reach its full sequential read/write potential.

There is no downside to running a faster SSD in an older slot - especially since you may eventually transfer the SSD to a newer laptop in the future. Apart from the bandwidth difference, there are no disadvantages to using an SSD that does not match the PCIe generation of the slot.