Re: my population growth freezes when playing online, usually between 400-500k s

Hi  - I haven't noticed a difference in population growth between playing online and offline. Is it possible that your design is different? You really need High Density / Low Wealth residential to bump population up that high. To increase population, you need the correct roads to upgrade the residential to HD, keep the residents happy by providing enough shopping and parks appropriate to their wealth level (as well as providing all the services like fire/water/garbage etc)

Are your roads upgraded to HD? If you've plopped a lot of High Wealth parks or increased your residential wealth by plopping certain buildings, you can limit the population capacity. HD/LW residential buildings house  a lot more sims than HD/HW residential. 

I would check your land value data map to make sure you have LW, check the happiness data map to see what, if anything, is bringing the happiness level down, and your density data map to see where you may need to upgrade roads to support HD residential. 

If you want to post pictures of your data map and population data, we can see if anyone has more ideas to help.