Would humanoid sized insectoid lifeforms be relatively slow considering that their exoskeleton would have to be rather heavy since it takes more 'armor' to get the same relative protection? Generally they are portrayed as very quick physically, but I think they would actually be slower, like a knight walking around; they would however likely be very strong.
Could someone use a little engineering or biology to support this?
Probably. Insectoids might be slower.
There are several issues though. When most people think "giant insects", they think of insects, only scaled up. Except, this is biologically and physically impossible. When you scale things up, the mass scales by the cube - you'll need very thick legs (think elephants) and most likely a standing posture. You'll also need a proper respiratory/cardiovascular system to transport all the extra oxygen required - not very insect-like if you ask me. Even the "giant insects" in the fossil record aren't so giant at all, they only have a large wingspan, and their body's cross-section is actually smaller than some of our giant beetles today.
But let's suppose we make those modifications and end up with something like a rhino with 6 legs. If we stopped here, then it would be about the same speed as a vertebrate of equivalent size. This is because the muscles involved are pretty much the same between insects and vertebrates, their strength is determined by their cross-section, which scales slower than mass. This is also one reason why bigger creatures are slower, relative to their body size. Insects are only fast (and strong) because they are small; there are plenty of fast, small non-insects like bats and lizards.
Now let's consider the protection. You mentioned "the same relative protection", but this quickly becomes impossible if you scale up too far. For physical reasons, materials become much weaker the bigger they are. Drop an ant from 100 times its height and it would walk away without a scratch. Do the same with a giant ant and it will crack like an egg. Our giant insect would need fancy materials for its exoskeleton to approach the same level of protection, maybe some form of metal (hey it's possible). But it still won't be of the same relative strength unless we start delving into cybernetics. And yes, the extra mass would make them even slower.
So to recap, for a realistic human-sized "insectoid", we'd have
Doesn't sound like an insect to me. I suggest handwaving and/or magic.
As @congusbongus says, it isn't an "insect", true, but it is a lifeform with an exoskeleton rather than an endoskeleton. You won't get any strength above that of an exoskeletal lifeform of equivalent size and biology, save for the usual tradeoffs between speed and strength that you get by adjusting the muscle-skeletal lever ratios - muscles have a maximum contraction speed, and a maximum force output and by adjusting the lever ratios, you can emphasise some combination of speed and strength.
An arthropod the size of a human can not normally exist. Hundreds of millions of years ago in the Carboniferous period, the giant millipede Arthropleura could reach 2.6 m and weigh as much as a human, but it lived in a time when there was a surplus of oxygen to support such a massive arthropod, and it had many pairs of legs to support its massive weight. Another extinct arthropod, Jaekelopterus could reach the same length and weight as Arthropleura, but it had the advantage of living in water, which eliminates most of the issues. A humanoid arthropod of this size would not fare well even in the Carboniferous. The exoskeleton would have to be very thick for it to be able to walk on only 2 or 4 legs, and in turn the muscles would have to be larger or more efficient. Furthermore, larger arthropods find it more and more difficult to molt, a human-sized arthropod would expend a very large amount of time and energy to shed its exoskeleton. Many things can go wrong while molting. For such an animal to realistically exist, it would need an atmosphere with a very high percentage of atmospheric oxygen, think upwards of 35%, an exoskeleton made of a very strong, lightweight material as opposed to chitin, which is what arthropod exoskeletons are normally made of, very large and efficient muscles, and a reliable way to distribute oxygen throughout the entirety of the animal's body. Even with all of these advancements, it would need at least 4 walking legs and would move very slowly.
