I worked on my secondary essays while my primary was still being approved so that by the time my primary was verified, I was able to turn in the secondaries within 1-2 days after receiving them.
If it was a weekend, I was able to finish around 1-3 secondaries a day (depending on how many essays they contained). On a weekday, I churned out about 1/2-1 secondary in the evenings after work. I always proofread the essays after writing, then proofread them again the day after to make sure I was completely satisfied (sometimes this resulted in additional changes/editing). After that, I considered them finished and moved on. I don't think it's necessary to obsess over secondaries like the PS.
Since many essays often have similar themes, the more secondaries you complete, the faster they usually go because you can recycle or reword a lot of previously written material.
Unlike my PS, I didn't have anyone else proofread my secondary essays.