Since these are potentially called in a tight loop, I think you can do this at comptime:

if (comptime builtin.is_test) {
    if (self.platform == null) return false;
}
// assume it's not-null in non-test.
return self.platform.?.inner.pumpMessageLoop(self.isolate, false);

I was wondering if I should use a different slower loop for these tasks separated from the microtasks one.
WDYT?

I don't understand what this is doing, so it's hard to say.

The page's messageLoopCallback is not being called very often. all of page.navigate (including processing all synchronous scripts) are executed before it's ever called. This could change if navigate became fully asynchronous though.

It's only being called often when we're "done" navigating and waiting for
1 - New cdp sessions
2 - I/O (i.e. an XHR request is live)

In both cases, I would think it's mostly a no-op.