Ever since revamping the tooltips to show buff % instead of multiplier, all buffs are additive. So +100% from meadhall +500% from swimming equals +600% (or 7 times the normal recruitment speed for a short period of time), rather than doubling the production speed (as some people probably expect).
Its worth noting that that also diminishes the value of the permanent production buffs in the form of the new buildings recently anounced (as +50% will add up to those 700% making it 750%, rather than 1050%), so I suggest careful consideration before you spend alot of dough on buildings that do barely anything