Hola gnzsoloyo desde ya muchas gracias por contestar. He seguidos tus consejos, que por cierto me han sido muy útil, ya que no tenía conocimientos de este recurso.
Como verás soy novato. Aquí te copio el resultado. Por lo que estube viendo parece estar bastante lento todo. Te comento que tengo indexada la fila dni de la tabla alumnos; id_act y dni de inscripciones; id_act, dni y asistencia de asistencia.
Código HTML:
Ver original<th class="draggable" data-column="id">id
</th><th class="draggable" data-column="select_type">select_type
</th><th class="draggable" data-column="table">table
</th><th class="draggable" data-column="type">type
</th><th class="draggable" data-column="possible_keys">possible_keys
</th><th class="draggable" data-column="key">key
</th><th class="draggable" data-column="key_len">key_len
</th><th class="draggable" data-column="ref">ref
</th><th class="draggable" data-column="rows">rows
</th><th class="draggable" data-column="Extra">Extra
<tr class="odd"><td class="right data not_null nowrap ">1
</td> <td class="data not_null ">SIMPLE
</td> <td class="data ">r2
</td> <td class="data ">ALL
</td> <td class="data ">indice standar
</td> <td class="data null"><i>NULL
</i></td><td class="data null"><i>NULL
</i></td><td class="data null"><i>NULL
</i></td><td class="right data nowrap ">12580
</td> <td class="data not_null ">Using temporary; Using filesort
</td> <tr class="even"><td class="right data not_null nowrap ">1
</td> <td class="data not_null ">SIMPLE
</td> <td class="data ">r3
</td> <td class="data ">ref
</td> <td class="data ">Indice Standar,id_act
</td> <td class="data ">Indice Standar
</td> <td class="data ">102
</td> <td class="data ">frural_programa.r2.dni
</td> <td class="right data nowrap ">1
</td> <td class="data not_null nowrap "></td></tr> <tr class="odd"><td class="right data not_null nowrap ">1
</td> <td class="data not_null ">SIMPLE
</td> <td class="data ">r1
</td> <td class="data ">eq_ref
</td> <td class="data ">PRIMARY
</td> <td class="data ">PRIMARY
</td> <td class="data ">frural_programa.r3.id_act
</td> <td class="right data nowrap ">1
</td> <td class="data not_null nowrap "></td></tr> <tr class="even"><td class="right data not_null nowrap ">1
</td> <td class="data not_null ">SIMPLE
</td> <td class="data ">r0
</td> <td class="data ">ref
</td> <td class="data ">indice standard,id_act,asistencia
</td> <td class="data ">indice standard
</td> <td class="data ">102
</td> <td class="data ">frural_programa.r3.dni
</td> <td class="right data nowrap ">4
</td> <td class="data not_null ">Using where
</td>