The style rule that handles transitions for the <<timed>> macro is based on the class selector .macro-timed-insert (from: core-macro.css at SugarCube v2 CSS > Built-in Stylesheets).
To change the duration of the transition for all <<timed>> invocations, simply add something like the following rule to your Story Stylesheet, which changes the duration from the default 400ms to 1s:
.macro-timed-insert {
transition-duration: 1s;
}
To change the duration of the transition only for select <<timed>> invocations, add something like the following rule to your Story Stylesheet, which changes the duration from the default 400ms to 1s for any <<timed>> which is wrapped within an element containing the class slow.
.slow .macro-timed-insert {
transition-duration: 1s;
}
And wrapping <<timed>>:
/* Using HTML. */
<span class="slow"><<timed … t8n>>…<</timed>></span>
/* Using custom styles markup. */
@@.slow;<<timed … t8n>>…<</timed>>@@