I'm very surprised it's organized as just a single 162 kB source document instead of being divided into smaller modules to make the code easier to work with, speed up build times, etc.

Were PDP-10 text editors of the day able to easily work with such large documents? And how long would it typically take to assemble all of that code in one go?

I don’t think TECO is inherently bad for this size of file. What might be tough is, if anything were compute-bound, and the processor were loaded down with other jobs, you could have the usual timeshare-related problems. If you wanted to look at a chunk of text, it would be a problem to send it to your TTY.

If you were using a KA10 variant, your process could get evicted pretty easily because the core memory requirements of all the running jobs had to be less than 256K words. The variants with paging would have a more efficient method for virtual memory.

Compile times would be affected by the same issues.