Your interpretation of the answer to that question is a bit wrong.

It would be an error to claim that delta wings don't perform well at low speed. Delta wings perform exceptionally well at low speeds.

The correct interpretation is delta wings are not fuel efficient. And this is true at low and high speeds.

There are advantages to delta wings at both high and low speeds. The following list are some of the known advantages:

1. Improved handling at high Mach numbers due to forward leading edge (note: this is the same advantage as swept wings but with the down side of being draggy when performing manoeuvres - swept wing planes retain much more kinetic energy after a turn).

2. Intrinsically strong structurally (it's a triangle!) thus can be built much lighter.

3. Retains quite a lot of rudder effectiveness at low speeds and high angles of attack (this is unintuitive because you'd think the wing would block airflow to the rudder but the strong vortex shedding on the leading edge maintains flow around the rudder).

4. Have very wide CG range compared to conventional aircraft. This is especially useful in bombers.

5. Have very benign and gentle stall behaviour. This is especially useful for paper airplanes since it can prolong flight at low speeds (paper airplanes simply dip their noses and regain speed instead of simply diving and crashing).

The major disadvantage is that it is draggy. Thus not fuel efficient. Thus in a business world where profit and cost matters delta wings are not attractive.

For supersonic flights the drag is apparently roughly on par with swept wings. Except, as mentioned above, due to the geometry of the wing it can be built lighter thus save fuel - so it sort of cancels out.

For toy planes, where efficiency is not of great concern (fun and "cool" is more important) delta wings are OK.

Still, if you use the same sheet of paper and built a long, narrow, cambered wing like this:

.. you'll find that the long narrow winged plane flies much further than the delta wing paper airplane given the same launch speed.

Your interpretation of the answer to that question is a bit wrong.

It would be an error to claim that delta wings don't perform well at low speed. Delta wings perform exceptionally well at low speeds.

The correct interpretation is delta wings are not fuel efficient. And this is true at low and high speeds.

There are advantages to delta wings at both high and low speeds. The following list are some of the known advantages:

1. Improved handling at high Mach numbers due to swept leading edge (note: this is the same advantage as swept wings but with the down side of being draggy when performing manoeuvres - swept wing planes retain much more kinetic energy after a turn).

2. Intrinsically strong structurally (it's a triangle!) thus can be built much lighter.

3. Retains quite a lot of rudder effectiveness at low speeds and high angles of attack (this is unintuitive because you'd think the wing would block airflow to the rudder but the strong vortex shedding on the leading edge maintains flow around the rudder).

4. Have very wide CG range compared to conventional aircraft. This is especially useful in bombers.

5. Have very benign and gentle stall behaviour. This is especially useful for paper airplanes since it can prolong flight at low speeds (paper airplanes simply dip their noses and regain speed instead of simply diving and crashing).

The major disadvantage is that it is draggy. Thus not fuel efficient. Thus in a business world where profit and cost matters delta wings are not attractive.

For supersonic flights the drag is apparently roughly on par with swept wings. Except, as mentioned above, due to the geometry of the wing it can be built lighter thus save fuel - so it sort of cancels out.

For toy planes, where efficiency is not of great concern (fun and "cool" is more important) delta wings are OK.

Still, if you use the same sheet of paper and built a long, narrow, cambered wing like this:

.. you'll find that the long narrow winged plane flies much further than the delta wing paper airplane given the same launch speed.