Slack, Teams, or Discord Uploads Slow

Why Uploads in Messaging Apps Can Be Slow

When you upload a file in Slack, Teams, or Discord, the data travels from your device to the app's cloud storage backend, gets processed, and is then delivered to recipients via a CDN. That is multiple network hops — each with its own potential bottleneck — before the file reaches anyone. Your upload speed is the first and most common constraint, but the path between your device and the app's storage can also introduce latency, regardless of how fast your connection is.

How Each App Handles File Uploads

Slack uploads files directly to Amazon S3 using a multipart upload process. Once the file is on S3, Slack's CDN delivers it to recipients — your upload speed determines how long the first step takes. For large files, the S3 multipart process breaks the file into chunks, which means a lossy connection causes individual chunk retries rather than a full restart, but each retry still adds time.

Microsoft Teams stores files in SharePoint or OneDrive depending on context (channel files go to SharePoint, chat attachments go to OneDrive). The upload goes through Microsoft 365 storage infrastructure, and recipients receive it via Microsoft's CDN. If your organization's SharePoint tenant is in a distant region, upload and download latency for large files can be higher than expected even on a fast connection.

Discord uploads attachments directly to its CDN infrastructure, which runs on Cloudflare. In practice, Discord uploads are usually fast if your connection to Cloudflare's nearest point of presence is good. If your ISP has poor peering with Cloudflare, or if you are on a VPN that routes through a distant exit node, Discord uploads specifically can feel slow while other services are fine.

What the Pattern Means

Fix 1: Confirm Whether Your Upload Speed Is the Bottleneck

Run a speed test while attempting a large upload in the messaging app. Watch the upload speed figure during the test — if it drops significantly compared to a test taken with no background activity, another process is competing for your upload bandwidth. If the speed test itself shows low upload even when the messaging app is paused, your ISP upload speed is the ceiling and no app setting will fix it.

As a rough guide: a 10 Mbps upload connection takes about 90 seconds to upload a 100 MB file under ideal conditions. Any network overhead, VPN encryption, or competing traffic increases that time. If your upload is 5 Mbps or lower, large file uploads and screen sharing will feel consistently slow regardless of which app you use.

Fix 2: Check Network Conditions Before Large Uploads

For large file uploads or important screen shares, use a wired Ethernet connection rather than Wi-Fi. Close other upload-intensive background apps: cloud backup clients (Backblaze, iCloud, Google Drive, OneDrive), Dropbox sync, video call apps running in the background, and any active torrent clients. Each of these competes for the same upload capacity. Pausing them immediately before an upload is the fastest way to confirm whether bandwidth contention is the issue.

Fix 3: Disable VPN or Request Split Tunneling

Work VPNs route all traffic — including Slack and Teams uploads — through a corporate gateway before it reaches the internet. That gateway may apply TLS inspection, malware scanning, and bandwidth policies that add significant latency and reduce throughput for large uploads. If policy allows, test one upload with the VPN disconnected. If speed improves substantially, ask your IT team about split tunneling the specific collaboration apps so their traffic goes directly to the internet without passing through the corporate gateway.

Fix 4: Use Link Sharing Instead of Re-uploading Large Files

For very large files — video exports, large design assets, raw footage — consider sharing a link from cloud storage instead of uploading the file again through the messaging app. In Slack, you can share a Google Drive or OneDrive link directly in a message. In Teams, the recommended workflow for large files is to upload to SharePoint first and share from there, which avoids re-uploading the same bytes through Teams' upload pipeline. This is especially useful when the file is already in cloud storage and re-uploading would just copy it through your limited upload connection again.

Fix 5: Fix Packet Loss Before Chasing Speed

Packet loss makes uploads retry individual chunks, stalls progress bars unpredictably, and wrecks screen sharing quality. If uploads fail or stall at random points rather than just moving slowly, run a packet loss test and switch to Ethernet. Wi-Fi packet loss caused by interference, distance, or a congested channel can look identical to a service-side problem but is entirely fixable on your end.

Frequently Asked Questions

Why are Slack, Teams, or Discord uploads slow when downloads are fine?

File uploads and screen sharing consume upload bandwidth. Most home connections have far less upload than download capacity, and VPN or security proxy inspection can slow collaboration app uploads further by adding processing overhead to every request.

Should I use the desktop app or browser?

Test both. If the desktop app is slow but the browser works, clear the app cache or reinstall. If both are slow, the bottleneck is your upload speed, VPN, proxy, or the service itself — not the client application.

Can packet loss affect file uploads?

Yes. Packet loss forces TCP retransmissions on each lost chunk, which can make progress bars stall or reset. It is especially damaging for live screen sharing and voice calls, where retransmitted packets arrive too late to be useful.