I've just started with go a few weeks ago, so take my info with a grain of salt.

Pocketbase uses a mutex for all the data reads and writes. That means that every write will block any readers for the write duration. If you do a lot of writes this will become quite inefficient quickly. You need some kinds of locking to write data, but if badly implemented it will pretty much roll back all concurrency advantages, you pile up goroutines that just wait to do something.

A good database does go very sophisticated steps to minimize locking, essentially you should expect from a modern database that most reads are lock free. Still it helps to understand how they coordinate locking and concurrency to optimize your applications. I.e. usually you design reads to be ultra fast and do writes in healthy bulk sizes which is also still fast but will net out to have minimum lock time. Slow reads should be done read only replicas or at times there isn't any load. So sqlite isn't slow/bad at all, if you use it correct, like any other decent database.

Below the sqlite WAL mode is explained. It is magnitudes more sophisticated then what pocketbase does. Pocketbase literally negates all that by their own locking strategy. It would likely be more efficient if the just remove their cache layer and fine tune sqlite for their use case.

https://sqlite.org/wal.html

Also if you require a very high amount of writes, that is realtime writes, as opposed to writes that can be delayed, you pretty much enter tough terrain. You need to make very deliberate choices and pay a lot of attention to system design depending on your requirements. So if something falls over from a high amount of writes, it because it's hard.