Chain tension should be so tight you cant pull it away from the bar.
thats too tight and you’ll risk warping or damaging your bar.
Sounds like a blunt saw and maybe your pinching it a little when you’re cutting? Dont forget to look after your bar too – take it off and file any burrs off the top and sides.
you should have got some files, give it a sharpen and again, dont forget to file the dogs down. ( the little bits infront of the cutting edge ) if they’re too small/big they can affect the cutting.
if all the above is fine, maybe its as suggested and something is blocked somewheres?