textoverlay timing

[James]

I've got a pipeline that previews and records 1080.
It has a clockoverlay and 9 textoverlay elements in the pipeline

I tried adding elements dynamically and statically having 9 elements with text of ""
There was little performance difference, so I went for static, as it was easier

A feature I wanted was to change font and point size. My whole page is same font and size.
Since you can't get a semaphore lock in a signal handler I do one line at a time with a very simple locking mechanism

producer creates a shm file with instructions for 1 line, then waits for file to not exist
consumer does work then removes the file

All works, but if I have all 9 lines of text then
[execute] new font or size
usually half the lines change and I need to repeat 2x or 3x before all are done. Since the result is not predictable it must be a timing issue.

If I manually issue commands line by line then all works perfectly, confirming timing problems.

What timing? Where can I find out about it?

Thanks
James