Does this slow anything down?

The 6.x-1.x branch has had (still has) some problems, especially with performance and caching. The 6.x-2.x branch is much better and doesn't actually slow anything down on the page request. It only includes a small amount of JavaScript and a timestamp for when the next cron run should happen. The JavaScript verifies if the current time is equal or later than the embedded timestamp, and creates a hidden AJAX request to run cron. This action should clear your page cache and make sure there is a new timestamp embedded in cached page's JS.

Keep in mind if you're using this module you should make sure to keep your cron run interval to no less than 12 hours, preferrably once a day since running cron causes your page cache to be cleared.

What is the difference between using this module vs programming cron tasks at the system level?
Does poormanscron sucking more resources?
Does the cache issue mentioned here applies to crontab activation too?

Running many sites (multi-sites install), I found this module very handy, avoiding dependencies at the system level.
I have the impression than cron delays page loading each time it runs. (I am very often stuck waiting for Drupal to get me to the wanted page)
If crontab avoid thats as well as cache flushing, I will give up Poormanscron reluctantly

thanks for clarification