Xt timers are non-preemptive; they do not operate like signal interrupts.
That is, they don't suspend execution of the client program at a specific
interval and call the timer procedure.

Rather, timers are handled by Xt as another form of input. The Xt event loop
watches for X events and for socket activity; the select() statement Xt uses
returns when the first timer is set to go off. Xt then calls any timers that
have expired.

The implementation means that Xt timers can't be used for real-time
operations.  If one callback takes some time to operate, for example, any
timers that "should have" gone off during its operation are not called until
the thread of execution has again returned to Xt's event loop.