Sadly no suggestions on this, so I’ll share what Drowned in Sound did in case any other forums encounter the same issue.
The following was added to the CSS:
iframe[src*="spotify.com"] {
max-height: 150px;
height:80px;
}
Which results in the embeds looking like this:
Those who faced the performance problems before have stopped complaining, so it seems that this fix was good enough to resolve things.