I am writing a function that takes derivatives of functions involving subscripts and an open-ended `Sum`

. It’s unreasonably slow. A minimal example (the real situation is much worse):

```
D(Sum(Exp(-(xx - Subscript(x, j))^2/(V + Subscript(Vx, j)^2)) Subscript(n, j), {j, nsp}), xx) // AbsoluteTiming
```

If I get rid of the `Subscript`

it gets better:

```
D(Sum(Exp(-(xx - x(j))^2/(V + Vx(j)^2)) n(j), {j, nsp}), xx) // AbsoluteTiming
```

but the `Sum`

is also contributing to the slowness. Focusing on the summand is much faster, with or without subscripts:

```
D(Exp(-(xx - Subscript(x, j))^2/(V + Subscript(Vx, j)^2)) Subscript(n, j), xx) // AbsoluteTiming
```

```
D(Exp(-(xx - x(j))^2/(V + Vx(j)^2)) n(j), xx) // AbsoluteTiming
```

Any ideas on how I get speed up this sort of operation? Not using subscripts is kind of off the table.