Back then, fitting in RAM was a big problem: The PDP-11 (the first kind of computer to run Unix, as opposed to UNICS) had a 16-bit address space, which gives you 64 K of RAM if you ignore the fact peripherals were memory-mapped and so took addresses away from actual memory. Later models had split I+D, or separate address spaces for Instructions and Data, but that's still only 128 K.

https://gunkies.org/wiki/PDP-11_Memory_Management

Later models could address 18 (256k) or 22 bits (4Mbytes) of memory.

Still restricted to I+D of 128K mapped in at any given time, plus your 8K of I/O space for bank 7.

I'd guess that back then system call overhead was lower than with modern CPUs with deep pipelines, speculation and their vulnerability workarounds.

I'm thinking most stuff was designed around single core performance. Lots of drivers had big'ole gigantic locks which really killed multi processor performance. These days with modern CPUs with a ton of cores the complexity has increased greatly.

It wasn't lower. But people put up with it.

Because they had to; there just wasn't a viable alternative

For sure, and up until the mid-to-late 80s RAM speeds were higher than processor speeds. So approaches to architecture were totally different.

Most minicomputer class processors were sewn together from multiple chips and transistors; even their register sets for the CPU were often not dissimilar from main memory. Texas Instrument's minicomputer (and later microcomputer) architecture even just put registers in RAM. In the microcomputer world, the 6502 got around having a very small register set by just having a 256-byte "zero page" with slightly faster cycle access than regular memory.

There was no need for complicated cache hierarchies. Relative cost of a context switch or interrupt or transition from user space to kernel etc. was way lower than now.

And the users of the system were by and large trusted. Security was more of a suggestion.

We did care about performance because 1 MHz was slow even back then and data bus were slow too. However we had to care more about memory size because we couldn't fit much in a few kB or 1 or 2 MB for huge machines. Finally, compilation time for substantial programs were... substantial! You could play games as in that sword fighting xkcd.