I bought a laptop with one 128GB M.2 SSD and one empty slot, then I got a 1TB SSD and installed it, then installed ubuntu on the 128GB and made the 1TB the home directory. Now I'm thinking, if I had installed both the Ubuntu system and the home directory on the 1TB, would that make the performance worse? Or make it better?
If both SSDs have the same read/write speeds, it would not matter for what you use one or the other. There can be significant speed differences though. To benchmark the read speed of a disk, you can run
sudo hdparm -t /dev/sdX
sdX with the actual name of your disk, as shown e.g. in the output of
However, if you have separate disks, you can access both at maximum speed independently at the same time, which gives you maybe up to twice the normal data rate, but only in the rare case where you have a large enough chunk of data to read from or write to both disks in parallel, which normally doesn't happen very often. It's hard to tell whether you would notice any difference during normal usage.
Another advantage of having
/home on the separate, larger SSD would be that you can easily back up only your complete data disk, or reinstall the system without messing with it (carefully though!). Also if the smaller system SSD dies sooner, your data would not be affected. It can happen the other way round as well, in that case you could restore your data backup and keep using it with the old system installation.