Performance regression with graal-based image

**Describe the bug**
When using graal docker image introduced in https://github.com/erdos/stencil/pull/53, .docx templates are rendered slower.

**To Reproduce**

1. download & extract a .docx template file into a `/tmp/templates` directory: [lipsum_sample.docx.zip](https://github.com/erdos/stencil/files/5440609/lipsum_sample.docx.zip)

2. Run two different containers, side by side (in separate Terminal windows):

```bash
docker run --rm --name stencil-0.3.10 --publish 5000:8080 --volume /tmp/templates:/templates ghcr.io/erdos/stencil:0.3.10
docker run --rm --name stencil-0.3.10-graal --publish 5001:8080 --volume /tmp/templates:/templates ghcr.io/erdos/stencil:0.3.10-graal
```

3. Run the tests:

* for regular container:

```bash
$ bash -c "for test in 1 2 3 4 5; do time curl -s localhost:5000/lipsum_sample.docx --header 'content-type: application/json' --data '{}' > /tmp/lipsum_sample_regular.docx; done"
real 0m10.385s
user 0m0.006s
sys 0m0.010s

real 0m4.191s
user 0m0.005s
sys 0m0.008s

real 0m4.245s
user 0m0.005s
sys 0m0.008s

real 0m4.239s
user 0m0.005s
sys 0m0.008s

real 0m4.263s
user 0m0.004s
sys 0m0.008s
$
```

* for graal container:

```bash
$ bash -c "for test in 1 2 3 4 5; do time curl -s localhost:5001/lipsum_sample.docx --header 'content-type: application/json' --data '{}' > /tmp/lipsum_sample_graal.docx; done"

real 0m19.201s
user 0m0.005s
sys 0m0.008s

real 0m12.841s
user 0m0.004s
sys 0m0.010s

real 0m13.111s
user 0m0.004s
sys 0m0.007s

real 0m12.936s
user 0m0.004s
sys 0m0.008s

real 0m13.117s
user 0m0.004s
sys 0m0.007s
$
```

4. Notice how graal-based image is consistently slower.

5. Stop the containers:

```bash
docker kill stencil-0.3.10
docker kill stencil-0.3.10-graal
```

**Expected behavior**
Performance of application in a graal-based image is on par with that of a regular one, or faster.

However, I know very little about GraalVM, as well as the goals to build a graalvm-based image. Perhaps, performance was not among the goals for this image.

**Screenshots**
No screenshots.

**Documents**
I've attached a link to the document above. It is a 800+ pages .docx file with no template placeholders in it.

**Office versions:**
Not applicable as far as I can see.

**Environment where template is rendered:**
- Stencil version: 0.3.10
- Java version: I used regular and graal stencil images.
- Operating System version: 10.14.6 (18G103)
- Docker version: 19.03.13, build 4484c46d9d

**Additional context**
No additional context.