motor - What happens if I reduce the air flow for an exhaust fan?

I'm not very familiar with 220V AC circuits and motors that run on them, so let me explain my question.

I'm building a soldering fume extractor for my workbench similar to what is shown in this video. It basically uses a simple ceiling exhaust fan along with some hose pipe to suck the fumes away from the desk to the window. He uses an exhaust fan which comes with a metal box enclosure and an outlet to attach the hose.

Unfortunately, where I live, I couldn't find the same type. So I settled for a simple exhaust and used the cardboard box it came in as the enclosure. I then connected 2 hose pipes which direct the air flow out to the window.

So this works all nice and dandy, but I noticed that when I close the box so that air only flows through the hoses, the fan speed slows down. I guess that's expected when the air flow reduces.

I'm sort of worried that this may put the fan motor under some additional stress and may cause it burn if I let it run for too long. Will this turn out to be some sort of safety hazard?

EDIT: I considered putting a fan regulator to the fan to control the speed, but I read online that adding a regulator to a single speed fan may lead to it getting burnt :(. So I guess that's not an option.

EDIT2: I also guess another option is to increase the dia of the hose or add more of them, but the ones I have are about 1 inch in dia and I'm not too keen on having more than 2 of them since it'll become unwieldy.