How did the temperature in the solution with a lower concentration of CuCl2 increased in a faster rate?
The reaction $\ce{Cu^2+ + 4 Cl- &lt;=&gt; CuCl4^2-}$ may lead to slowing down reaction with aluminium, decreasing the equilibrium concentration of $\ce{Cu^2+}$ in $\pu{0.8 M}$ $\ce{CuCl2}$. That all would lead to lower peak temperature due bigger heat dissipation before reaching maximal temperature.

Contributing factors may be


decreasing of chemical activity of water for the electrode surface reaction $\ce{Al(s) + 6 H2O -&gt; [Al(H2O)6]^3+(aq) + 3 e-}$. These  electrons, charging the electrode, then reduce copper: $\ce{[Cu(H2O)n]^2+(aq) + 2 e- -&gt; Cu(s) + n H2O}$
increasing of viscosity, therefore decreasing of diffusion coefficients and another decreasing of the reaction rate.