I have found the solution to this problem.
I have a Gigabyte B450M motherboard and it turned out that Bios F32 was messing things up.
I upgraded to Bios F40 and now the Radeon 5700 works at full load and max clock frequency at all times in games with very good performances.
Therefore, if you have this motherboard, do not stop at Bios F32 when upgrading, it will make your Rx5700 perform way below expectations.
Instead upgrade all the way to Bios F40 ( you must install Bios F32 before F40 anyway, it is necessary) which alternatively will make your motherboard compatible with the new Ryzen 3600 CPU.