Slow https(SSL) downloads and/or high CPU usage during download?
Most websites are using https by todays standards.
Modern SSL Ciphers can result in very high CPU usage and also slow speed - especially on older/single-core ARM CPUs commonly e.g. used in older/smaller (e.g. Synology) NAS systems.
Most likely the CPU/Environment doesn't support hardware accelerated encryption! Especially modern AES GCM Cipher also suffers from known performance issues in older Java versions.
If you have an Intel/AMD CPU, you can experiment with newer Java versions (eg recent Java 1.8 or latest LTS version).
All in all there isn't much you can do but for some plugins, you will find a workaround setting which lets you disable https for a particular service.
See Settings --> Plugins --> yourservice.com --> ENABLE "Prefer http over https" (or similar) or DISABLE "Use SSL/https" (or similar)
If your JD is running on a Synology NAS you might want to check out our Support forum thread regarding Synology SSL issues.