Suppose I have generated the following QRCompactWYQ:

```
Q = qr(randn(500, 10)).Q;
```

and I want to do in-place multiplication of Q with

```
x = randn(500)
```

to

```
y = randn(500)
```

i.e.

```
@btime mul!(y, Q, x)
```

This runs in **2.9s** in my machine, which is significantly slower than doing

```
@btime y = Q*x
```

which runs in **8.8μs**.