Experiment to allow animating fillOpacity of Rect using native driver.

import React, { Component } from 'react';
import { StyleSheet, View, Dimensions, Animated } from 'react-native';
import { Svg, Rect } from 'react-native-svg';

const { width, height } = Dimensions.get('window');
const AnimatedRect = Animated.createAnimatedComponent(Rect);

export default class App extends Component {
  state = {
    anim: new Animated.Value(0),
  };

  componentDidMount() {
    Animated.timing(this.state.anim, {
      toValue: 1,
      duration: 3000,
      useNativeDriver: true,
    }).start();
  }

  render() {
    const { anim } = this.state;
    return (
      <View style={styles.container}>
        <Svg width={width} height={height} viewBox="0 0 100 100">
          <AnimatedRect
            x="5"
            y="5"
            width="90"
            height="90"
            fill="green"
            fillOpacity={anim}
          />
        </Svg>
      </View>
    );
  }
}

const styles = StyleSheet.create({
  container: {
    backgroundColor: '#ecf0f1',
  },
});

lib/attributes.js

@@ -220,8 +220,8 @@ const RectAttributes = {
     ...RenderableAttributes,
     x: true,
     y: true,
-    width: true,
-    height: true,
+    rectwidth: true,
+    rectheight: true,
     rx: true,
     ry: true,
 };