Pixi: Poor page experience (High TBT/FID)

URL
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https://www.gaborshoes.co.uk/listings-ssr.html

Details
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| Metric | Field data | Lab data |
|-----------------|------------|---------|
| LCP | n/a | 4.6 s |
| FID | n/a | 6346 ms |
| CLS | n/a | 0.01 |
| HTTPS | pass | --- |
| Safe browsing | pass | --- |
| Mobile-friendliness | pass | --- |
| Intrusive Interstitials | | --- |

Notes
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Components in use: amp-access, amp-analytics, amp-animation, amp-bind, amp-consent, amp-form, amp-geo, amp-iframe, amp-install-serviceworker, amp-lightbox, amp-list, amp-mustache, amp-position-observer, amp-sidebar

This is a product listings page that has been generated via the PHP AMP Optimizer - it is the same site as the page discussed in https://github.com/ampproject/wg-performance/issues/61. Therefore, we're aware of issues that have led to the slow LCP time and we are working on optimisations for those currently so that part can be ignored for now. However, we were looking for some advice on why the TBT/FID score is so high, and if there's anything we can do in terms of how we have implemented AMP to improve this.

/cc @ampproject/wg-performance

Start with the generated page at https://www.gaborshoes.co.uk/listings-ssr.html and the listed AMP components; reproduce the reported lab metrics and inspect the main-thread work behind TBT/FID. Compare the implementation with wg-performance issue 61 and document concrete AMP-related causes or changes, with completion marked by an explained cause and actionable recommendations.

Written by the indexing model from the issue text.