Why Do Bagels Take Longer To Toast Than Regular Bread?
Toasting is a form of browning, and browning requires temperatures in the neighborhood of 300 F (150 C…see Maillard reaction). There are at least two things that impede this progress when toasting a bread product: mass, and water.
The more mass the bread has, the more heat is required to be absorbed to raise the temperature of the bread by some amount. So naturally, a thicker, often denser bagel will require more heat to toast than a thinner, less-dense slice of sandwich bread.
Of course, a significant amount of the mass is water, and water at the surface of the bagel or other bread will prevent the temperature of the bread from reaching the level needed for browning. Heat must be applied long enough to drive off enough water to allow the temperature to rise high enough; there is more water stored in the thicker, denser bagel than in regular bread slices, and it takes time for enough of this water to work its way out of the bread that the surface can toast. †
Of course, a bagel also has a thick, relatively impermeable side which limits how much water can leave that side. So as the bagel heats up, moisture which is being driven off preferentially exits the sliced side. So not only is there more moisture to remove, the moisture isn't as free to move as it would be for sliced sandwich bread.
In other words, bagels need more total heat input to achieve the browning that is &quot;toasting&quot;.
A typical toaster does not provide a way to control the level of heat during toasting. The main parameter that can be adjusted to account for toasting is time. I.e. to achieve the same total amount of heat input and subsequent toasting, an increased amount of time is required, because the same level of heat is being applied.
It is likely that a toaster could be designed that uses a variable amount of heat output according to the toasting setting, and which could toast a bagel in the same amount of time as thinner sandwich bread slices. But a higher heat level offers a much faster toasting over all and thus a narrower range of time for the right level of toasting. I.e. it'd be a lot easier to apply too much heat and burn the bread. The heat level for toasters seems to be designed to provide a good, user-friendly speed of browning, so that the casual user can easily achieve the desired level of browning without too much variability and over-toasted bread.

† Note: one might assume that because of the boiling step used to cook bagels, they might absorb additional water during that step and thus have more water to be removed during toasting. A logical assumption, but one which turns out to be incorrect. Little or no water is absorbed during the very brief boiling of the bagel:

Boiling breads like bagels and pretzels effectively sets the crust before it goes in the oven. The water doesn’t actually penetrate very far into the bread because the starch on the exterior quickly gels and forms a barrier.


The starch on the exterior quickly gels and forms a barrier. This keeps the water from penetrating very far into the bread.


no, the type of water used for boiling has no effect on the flavor because the dough doesn’t absorb any liquid as its surface hardens.