In order to streamline things and make things more reasonable, there's an easy solution:
Create a master matrix of Pop traits to jobs. 'x trait makes you .4 better at y job'. This is very easy to do in an objective manner. Communal makes you a little better at every job. Strong makes you a better worker. Every pop race then can have a value calculated for it's attraction to a given job. Every planet then gets an average attraction level for each pop type, based off of the jobs already on that planet, and the habitability of that planet for that pop.
Since a city planet or habitat or Gaia world has 100% habitability, and differences between most jobs are small, many wotlds will still be very cosmopolitan. But the science world is a world of intelligent species, thank you very much.
You get rid of the current job weight mechanic. When a new pop enters the work force either because its place of employment was demolished or because it's a new pop, it takes the highest strata job available.
When a new job is opened, it's filled by whatever pop has the highest aptitude for it, but only if that pop is of equal or lower strata and and only if it's aptitude for it's new job is higher than it's old one.
This means that a newly unemployed/created pop is going to have very bad options. But since specialist and ruler jobs are constantly being created, pops will be able to bubble up through the ranks if they're high aptitude.
Finally, if jobs are eliminated, the least apt pops with that job are released into the workforce first.
All in all this leads to much more naturally pleasing worlds. But the other benefit is that it relies on event based calculations. When a planet produces a pop, it assigns a job, then calculates the growth weight of all pops that can grow there. When a building is demolished, a dozen pops recalculate their position.
No constant recalculations required.
Obviously this is just my 5 cents, but I think this would fix most of my problems with the game, from a design and performance perspective. (Balance and bugs are another matter)
Would make the appearance of a new race (via Gene/robo modding) a pretty hectic experience though.