Why network speed drops during an encrypted transfer

A download or upload gets slower when you turn on a VPN, use an encrypted file transfer, or connect to a work network. It can feel as if encryption is slowing the whole internet, but the cause might be the route, the server, your device, or another app using the connection.

Compare your normal connection with the transfer running, then change one factor at a time. The Speed Test measures download and upload speed, ping, and jitter so you can see whether the slowdown affects more than one app.

Compare the connection with and without the transfer

1. Open the Speed Test on the same device and network you use for the transfer. If the transfer normally uses a VPN, keep the VPN connected for this first measurement.
2. Run the test with the transfer paused. Record the download speed, upload speed, ping, and jitter. Run it two or three times and use the pattern, not just the highest result.
3. Start the encrypted transfer and let its speed settle for a minute. Note whether it is downloading or uploading, and compare its rate with the idle test.
4. While the transfer is still running, run the Speed Test again. This test competes for bandwidth, so it shows what capacity remains rather than the transfer's exact speed.
5. If it is safe and allowed, repeat the comparison with the VPN disconnected. Do not send private work or sensitive information without the protection your organization requires.

Check the units before comparing results. A file transfer may show megabytes per second, written as MB/s, while a speed test usually reports megabits per second, written as Mbps. One MB/s is about 8 Mbps, before accounting for transfer overhead.

What the results mean

The speed test also slows during the transfer

The transfer is using much of your available bandwidth, or another shared limit is affecting all traffic. A large upload can also slow other activities even if download speed is high. Check for cloud backups, game updates, video streams, or other devices transferring data at the same time.

The transfer is slow, but the speed test stays fast

Your internet connection may have spare capacity, but the transfer itself may be limited by its server, the remote network, the transfer app, or a single connection between the two ends. A fast speed test cannot prove that every file server can send data at the same rate.

Speed drops only while the VPN is connected

The VPN may use a longer or busier route, a slower server, or a protocol that takes more work on your device. A VPN also adds extra data around each packet. If the drop is large, compare another nearby VPN location or ask your workplace or VPN provider which settings are supported.

Only one device or Wi-Fi location has the problem

Look for a device-specific limit, weak or inconsistent Wi-Fi, or background work on that computer. If other devices stay fast in the same room, the router or internet plan is less likely to be the only cause.

How to fix the slowdown

1. Check for competing traffic. Pause cloud sync, large downloads, and other uploads, then test again. An upload can fill the connection and make browsing or calls feel slow even when the download rate looks fine.
2. Compare a different VPN route. If you are allowed to change it, try a nearby VPN server or another protocol recommended by the provider. On Windows 11, check Settings › Network & internet › VPN. On macOS, check System Settings › VPN. Do not weaken encryption or bypass a required work VPN just to increase speed.
3. Check the device while transferring. Encryption and file handling can use processor time, especially on an older or busy device. Close unnecessary apps and retry while the computer is plugged in. On Windows, open Task Manager with Ctrl + Shift + Esc; on macOS, open Activity Monitor from Applications › Utilities and look for sustained high processor use.
4. Test a wired connection or a better Wi-Fi position. Connect Ethernet if available, or move closer to the router and test again. Windows 11 has Wi-Fi options under Settings › Network & internet › Wi-Fi; macOS uses System Settings › Wi-Fi. The Wi-Fi Signal tool can compare response time, jitter, and lost requests in different locations. It does not read Wi-Fi signal strength in dBm.
5. Compare another transfer source. Try a different file or server using the same app, or the same source from another device. If only one server is slow, its load or route may be the limit. If every source is slow only on this device, check the app, operating system, and device workload.

Why encryption can affect transfer speed

Encryption changes data into a form that cannot be read without the right key. Current computers usually handle common encryption quickly, so ordinary HTTPS browsing often has little noticeable cost. A VPN can have a larger effect because it encrypts traffic and sends it through another server. The extra route can add delay, and the encryption work can matter on a low-powered or heavily loaded device.

Some transfers also use one connection at a time. That connection may not fill a fast internet link, especially if the server is far away or responds slowly. A speed test can use a different route and method, so its result is a comparison point, not a guarantee of the speed a particular encrypted transfer should reach.

TooLab's Speed Test exchanges test data with our server to measure the connection. Compare results under similar conditions, and avoid running a speed test during a transfer you need to keep uninterrupted.