# Plot manipulation too slow

I tried manipulate simultaneously the plot of a function and its indefinite integral from 0. However, the evaluation is too slow and I believe that's not because of my computer hardware. Is there any way to optimize it?

f[x_] := (3/2) Sin[((2/10) x)^2];
i[y_] := Integrate[f[x], {x, 0, y}];
label1 = "f(x) = \!$$\*FractionBox[\(3$$, $$2$$]\) \
sin(\!$$\*FractionBox[SuperscriptBox[\(x$$, $$2$$], $$25$$]\))";
label2 = "\!$$\*SuperscriptBox[SubscriptBox[\(\[Integral]$$, $$0$$], \
$$x$$]\) f(u) \[DifferentialD]u";

Manipulate[
Plot[{f[a], i[a], 0 a}, {a, 0, x},
PlotRange -> {{0, 15}, {-2, 8}},
Filling -> {1 -> {3}, Axis},
PlotLegends -> {label1, label2}],
{x, 0.1, 15, .2}]

• i[a] is being reintegrated at each time step. Try With[{ia=i[a]}, Manipulate[..., {f[a], ia, ...] ] instead. Works great for me. – b3m2a1 Mar 18 '17 at 20:17
• Just worked fine! – Fred Mar 18 '17 at 20:19
• You should also look through this for a more thorough discussion mathematica.stackexchange.com/a/28920/38205 – b3m2a1 Mar 18 '17 at 20:21

The simple solution to your problem is to integrate f just once before making a down-value (internal definition) of i. You can accomplish that by replacing SetDelayed ( := ) in your definition of i with Set ( = ). With this approach, plotting i becomes a simple expression evaluation in the Manipulate rather than a symbolic integration each time you move the slider for x.
f[x_] := (3/2) Sin[((2/10) x)^2] 