I've discussed this question with Mendix support and it seems that there is no solution for this issue. The Signature widget has not been updated since last 3 years and is recommended to use a new one (Signature) from the app store (this is possible if you use Mendix 7.13 or above versions of Mendix).
This widget is based on React and should have a better performance.
The solution provided by Mendix support and this solved the delay.
The lag is happening because Mendix hybrid apps on iOS take a screenshot of the page on every touch. It does this to show a 'nice' swipe to go back to animation. The side effect of this is that taking that screenshot completely blocks the user interface for 100-200 milliseconds. Therefore the workaround is to remove the following line from the file config.xml that makes Cordova plugin to take the screenshots:
<plugin name="com.darktalker.cordova.screenshot" source="npm" spec="0.1.6" />
After that, the app should be much more responsive to user interaction and the lag when drawing the signature should disappear.