Big Performance Issue in 3.3.0 compared to 3.1.0

* [x] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## What did you do?

Created a line chart with **14 Datasets** and **~3200** entries divided across those 14 data sets. The chart has a gradient fill underneath. The device was running iOS 12.2. Drew the line on screen and started zooming in and out and panning.

## What did you expect to happen?

Consistent performance between version 3.3.0 and the older version 3.1.0

## What happened instead?

Performance dropped considerably. Computation and drawing of the chart slowed the entire app down indicating a blocking of the main thread. The same happened when zooming and panning. Only when zooming in very very close the performance started to pick up and the panning became smoother. I reverted back to version 3.1.0 and there were no performance issues whatsoever with the same dataset!

## Charts Environment

**Charts version/Branch/Commit Number:** 3.3.0
**Xcode version:** 10.2.1
**Swift version:** 5.0.1
**Platform(s) running Charts:** iOS
**macOS version running Xcode:** 10.14.4