Does SSD size affect speed in the Mac mini?

Sequential write performance between the SSD sizes in the Mac mini 2018:

128GB -> ~0.6 GB/s

256GB -> ~1.3 GB/s

512GB -> ~1.9 GB/s

1&2TB -> ~2.6 GB/s

Reads are all > 2 GB/s (2TB 2.6 GB/s).

https://www.reddit.com/r/macmini/comments/9vibhk/mac_mini_2018_ssd_speed/

If you're asking will a faster primary replacement disk (system disk) allow for faster system performance, yes.

If you add another internal SSD or an external SSD without replacing the primary system disk with a faster one, no.

Additional storage alone does not increase system performance, it may allow for faster data transfers though.