In general adding ingredients to any mix will decrease its drainage.
Consider a jar of marbles. It has about 30% air volume, and the spaces between are large so offer little resistance to flow.
Consider a jar of BB's. Also spherical components so also about 30% air volume. The passages are smaller so their is some increase in flow resistance.
Now mix them.
The result is a system where water has to drain between the BBs that are between the marbles. There are fewer paths because the marbles take up some of the room.
To increase the drainage of a mix you have to add chunks that internally have better drainage than the mix you are starting with. This can mean that NOT mixing the two components well will improve drainage. Consider the marbles and BB's, having it in layers may have better drainage than having them separate. This works only if the smaller stuff can't move into the voids in the larger stuff.
This can be used to improve drainage in pots: Put a layer of fast draining material that covers the drainage holes, a layer of landscape cloth, and your mix. The drainage holes of the pot are no longer a choke point for drainage flow.
In general a mix of sand and clay will have poorer drainage than will either one alone.
Soil structure is important to drainage. Adding organic matter to soil improves the structure over the long haul because it will decompose leaving tiny channels between the lumps. Humic acids from that decompostion help clay maintain some structure too, due to lowered pH.