Previously my Japanese chlorine tablets dissolved with no problem at all.

Now I think with the Chinese ones (Poolchor?), blue flecks, they all seem to go into a solidified mass, not dissolving quickly.

Has anybody else had such experiences, and if so what is now their preferred choice of tablets, performance and cost wise?

I buy tablet form from a water filter company for m wife to use when I'm not around, I use 90% powder from lazada. Cheap but you just take extra care. 

The Chinese have developed an unenviable reputation for shoddy goods. It's a bit like when the Japanese starting making cars for the American auto market.

I'm betting the Chinese tablets are mostly cheap inert filler.

The main thing is the check the Cl levels after a few hours in the water. If you are getting sensible readings it doesn't matter how long they take to dissolve.

I do my chlorination - in addition to the long term algae protection by copper sulphate - once a week (if I don't forget) by a handful of common chlorine (the one without cyanuric acid) thrown in OF tank where from it's slowly taken by pump, distributed into pool. 

 

With this simple procedure I have clear water every day throughout the year.