We're planning to talk about Encumbrance more in depth in a later blog post. But the gist is that it's weight based, not slot based, so, indeed, 1 suit of armor may be more encumbering than 50 different alchemical components. This was essential, as, among other reasons, variable-Quality items probably wouldn't have stacked under a slot-based system in a way that wasn't confusing as hell.
You will be able to buy greater personal encumbrance as a Strength-based feat set (in addition to equipping bags) and, indeed, going over various Encumbrance thresholds will slow you down.