— size of hole..
door closer manufacturer provided rather thick screws. (I think Dorma also provided bigger screws of similar shape.)

However, years ago I used smaller holes on door frame, and I can see other door contractors using smaller holes. I think thinner screw are more likely to break [1].

So how did I drill a bigger hole? I had to start with the smallest drill bit and end with the full size. (If I start with a bigger drill bit, it simply fail to make progress.) I had to buy many drill bits of various thickness.

Therefore if the final full size is 2mm rather than 6mm, then drilling time (and effort) would be much better than

— slow with firm pressure, without bending.

[1] If any other drill bit is driving at a slight angle to the existing hole, then it is fine, but a small drill bit could bend and break if under stress. (If you drive at an angle, then be gentle.)

The rotation needs to slow down enough to make a difference. The cordless drill proved better at slow drilling.

— Slow progress, but better experience than drilling into hard concrete (with embedded stones). Less strenuous. Less tiring. Cordless drill is often enough.

— quality of bit .. I now want the highest quality. The regular or standard quality won’t make a difference.