By default, Opera (like many Chromium-based browsers) may only use a single connection for a download. If that connection hits a speed bottleneck, the entire download slows down. Parallel downloading bypasses this by ensuring that if one stream is slow, the others can still maximize your connection's throughput. How to Enable Parallel Downloading in Opera
Turbocharge Your Browsing: A Guide to Opera’s Parallel Downloading Flag opera //flags/#enable-parallel-downloading