Re. etch speed: Etching is an oxidative reaction in which the metallic copper is oxidized. Thus in spray etching, the ferric chloride can be considered saturated with air/oxygen and works faster at lower temperature than bath etching in which not only may the etchant not be saturated with air, but there is a laminar layer on the copper surface. Gentle rocking helps, but so does heat. As a rule of thumb, reaction rate doubles for every increase in temperature of 10°C. I etch in a bath at 40° to 60°C. It generally take about 15 minutes. There is an "edge" effect, which means the edges will etch faster than the center of the board, and fine details (remember viscosity and laminar layer) will etch slower. On occasion, I will use a soft sponge with the etchant to carefully rub over those areas.
Re. your resist: What wavelength is your laser? Presumably near IR? I would consider a dye that absorbed that wavelength in preference to a pigment. I would also try to find materials that ablated without leaving an ash. Some "paints" will be worse than others. Acrylics and cellulose-like lacquers (e.g, clear model dope) are probably better than enamels. I would not use water-based latex paints.
After ablating, the etchant should wet the areas to be etched and tend to bead up on the painted areas. If you don't see the wetting, you probably have some residual resist. The initial etching reaction is very fast. I use a photoresist. Sometimes, after development, I may doubt whether more development is necessary. I very quick dip in or drop of ferric chloride will turn clean copper a salmon color. If I don't see wetting or the color, I develop a little longer. As you get your system standardized, that will rarely be an issue, but when starting out, it is a good test and a lot quicker than waiting for hours to find almost no etching.
John