Serving Rails static assets is hard, especially on Heroku Cedar where Heroku forces Rails to serve static assets itself (which isn't particularly performant nor worth your dyno-dollar)---this is different than Heroku Bamboo which used Varnish and is no more. To make the best of the situation, I highly recomend:
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Using the Heroku-Deflater gem which will selectively gzips assets (it gzips text-based files; and excludes images or binary files, which can actually be bigger when gzipped)
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Configure your production environment to set cache-control headers and get out of denial about how static assets are being served on Heroku Cedar
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Use AWS Cloudfront (or a CDN of your choosing) to serve the assets. Cloudfront is great because you can use the same Distribution