Almost every software timer follows this rule (even ones in the kernel) - "Interval timers / Thread Sleep guarantee that the amount of time waited is at least t

Almost every software timer follows this rule (even ones in the kernel) - "Interval timers / Thread Sleep guarantee that the amount of time waited is at least this long, it doesn't guarantee that it is exactly this long. As seanalltogether mentions, timers are run at the beginning of the render loop => "Run timers, layout page, render page, repeat". If you're busy doing stuff in #2 or #3 (or if your timer handler takes forever), you're not running #1.