time = TimeUsed[];
Simplify[-((3 t)/4) - 3/8 t Sin[ x[1] + x[2] + x[3] +... + x[18] + x[19] + x[20]] + 3/8 (2 + Sin[x[1] + x[2] + x[3] + ...+ x[18] + x[19] + x[20]])];
TimeUsed[] - time
results: 0.07
time = TimeUsed[];
Simplify[-((3 t)/4) - 3/8 t Sin[ x[1] + x[2] + x[3] +... + x[13] + x[14] + x[15]] + 3/8 (2 + Sin[x[1] + x[2] + x[3] + ...+ x[13] + x[14] + x[15]])];
TimeUsed[] - time
results: 22.2
The second procedure is obviously more simple than the first procedure, why the running time of the second procedure (i.e. 22s) is much more than the first procedure (i.e. 0.07), and how to improve the second procedure?