Building a responsive webmail frontend: Should I cache message bodies or always fetch them live?

Looking for advice on building a responsive webmail frontend (Kerio Connect + SOGo) I'm building an internal webmail application for our company and would love to hear from anyone who has worked with Kerio Connect, SOGo, Dovecot, Exchange, Zimbra, or similar mail systems. Current Architecture Backend: Next.js (Node.js) Mail Server: Kerio Connect Webmail: SOGo Message list is synchronized and cached in PostgreSQL. Message bodies are fetched live from SOGo. SOGo sessions are persisted and reused successfully, so I'm not logging in on every request. Current flow: Frontend ↓ Next.js API ↓ Reuse persisted SOGo session ↓ Fetch HTML body from SOGo ↓ Return HTML to frontend Current Issues 1. Opening an email is noticeably slower than loading the message list The message list loads almost instantly because it comes from PostgreSQL. However, every time a user opens an email, the application must: Send a request to SOGo Retrieve the HTML body Return it to the frontend Render the HTML Even reopening the same email a few minutes later triggers another request to SOGo. 2. No message body caching I intentionally chose not to cache message bodies because I wanted every email to be fetched directly from the mail server. As a result: Reopening the same email always makes another request. Switching to another email and back makes another request. Opening an email from days ago also makes another request. I'm starting to question whether this is the right architectural decision. 3. Large HTML emails are significantly slower Simple emails load quickly. However, emails containing: Outlook signatures Inline images Large HTML tables Complex HTML layouts take noticeably longer to open. I'm trying to determine where the bottleneck actually is: SOGo generating the HTML? Network latency? Backend processing/sanitizing HTML? Browser rendering? 4. Some redundant database lookups During a single body request, my application performs multiple database lookups for credentials and session information. These can probably be optimized or memoized, although I don't believe they're the primary performance issue. Questions Do webmail clients like Gmail, Outlook Web, Roundcube, or Thunderbird cache message bodies after the first time they're opened? If you were building a webmail client on top of Kerio/SOGo/IMAP, would you: Always fetch the body live? Cache message bodies? Use a hybrid approach? For large HTML emails, where is the bottleneck usually found? Mail server Network Backend HTML processing Browser rendering Are there any well-known architectural patterns for building a responsive webmail frontend on top of an existing mail server? Constraints I don't want to modify Kerio Connect. I'd prefer not to permanently store message bodies unless necessary. SOGo sessions are already persisted and reused. My goal is to make opening emails feel as responsive as Gmail or Outlook while keeping the architecture relatively simple. I'm mainly looking for architecture and design advice rather than framework-specific solutions. If you've built or optimized a production webmail client before, I'd really appreciate hearing how you approached message body retrieval and caching.