Slow drain after tub-to-shower conversion
If you didn't take out a trap it is likely in the ground, and the shown section added a second trap, having two traps in line is the problem, it is called double trapping.

The air in between the two traps is stuck and requires some pressure to clear and start the water flowing, provided by your build up of water in the shower.  This region between the traps is also more likely to accumulate material and clog, which is likely why it is getting worse.

It can be fixed by venting between the traps, or removing one of them, but since they are both in concrete it is not an easy fix.  Short term you could try and snake the drain or use other methods to try and clear debris to improve drainage, but it is going to be a problem.