Try putting some Javex in there and leave it overnight. Put it in when it's almost done flushing that way some will get sucked into the P trap and some of the rest of the plumbing.
Double check what those tablets are made of though, make sure it's not ammonia. I don't think so though, pretty sure they are bleach based.
Though when you see it flushes slow, is the water rising up and then it takes long to flush, or is it that there's not enough of a rush of water when you flush? If there's not enough water going into the bowl in first place then your problem is upstream, possibly the holes in the bowl where the water comes out or the flap in the tank is not opening enough.