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It is possible that your CSS is being loaded after your JavaScript, depending of the size of your CSS and the event which trigger the style to change you could experience some minor delay.
You could try to load your CSS in the header before your actual JavaScript and use Chrome Network tools to see if you have any bottleneck when loading your CSS.
Further tips to optimize your CSS delivery are:
- Minimize HTTP Requests, use CSS concatenation.
- Enable compression, consider gzip.
- Minificate your CSS, to reduce CSS file size.
- Enable browser caching.
- Optimize Images and SVG.
Source:stackexchange.com