The core book seems rather terse on the shield rules, which is to be expected from a game that advertises itself as streamlined. That being said, there are certain logical considerations that can be used to get an idea whether the action would be relatively fast or slow.
The key is the question: which type of shield are you using, and do you want to distinguish them? The terminology can be slippery and inconsistent between times and regions, so I'll explain the possibilities in plain words.
- Single-handle-in-the-middle shields, sometimes (but not universally) called bucklers. These should be treated as weapons in all senses: they're about as easy to pick up as a 'normal' weapon1, and the shield-hand grip is all they need to be ready. These are likely to be the smallest shields (but not always - see the scutum as a counterexample). The realistic downside is that they make it harder to put all your weight behind a push/rush/charge, but of course the core-book don't account for it - it just gives a flat strength+4 bash, regardless of even shield size.
- Double handle for a forearm/shoulder support, ideally with straps for extra stability. These may warrant counting as a readying of two items if you want to be realistic. Especially if one wants to fasten the straps (not doing so would deny the benefits of the firmer grip . . . but that's below the granularity of the core book's rules). The largest shields are most likely to be of this design.
- A 'hands-free' shield controlled by the neck guige rather than by an arm (not to be confused with just stowing the shield using the guige when not in use). This is a style that seems to be less known among non-armour-enthusiasts, and its use is counter-intuitive. Readying should be either as slow as the strapped shield, or slightly slower, the parry bonus less than for a arm-controlled shield as a downside of this grip style - that's the cost for having both arms available for wielding a weapon or two. But again, of course this is beyond the granularity of the rules; I've only seen this style handled by the rule-set of a single RPG.
If you don't want to over-complicate things, I suggest just treating shields as a regular weapon (the first option), and glossing over the question of straps for the shoulder-supported shields.
1 I say a 'normal' weapon because a realistic technique of wielding a shield absolutely does treat it as a weapon in all ways that matter: keeping it mobile, using it for the occasional bash or to influence the enemy weapon, making defensive feints etc.; don't mistake the Hollywoodian image of a shield as an easily outmanoeuvred stationary wall for the real thing (it comes from an overly literal interpretation of shield wall tactics applied to one-on-one fights).