Why would YouTube load slowly on every device here?

Since this follows the home network across different devices but disappears on other networks, focus on the router, its settings, or the connection between the router and your provider. A device-by-device fix is unlikely to explain that pattern.

When it happens, compare YouTube on a wired computer, if available, with a Wi-Fi device. Also note whether the issue affects every YouTube feature or just video playback. That can help narrow down whether this is Wi-Fi related or something specific to how video traffic is reaching the network.

Changing DNS is a reasonable test, but it does not rule out every routing or connection problem. If your router supports it, try a temporary test with IPv6 fully enabled, then fully disabled, rather than changing several settings at once. Keep notes so you can undo the changes.

Look for router features that inspect, filter, prioritize, or limit traffic, such as parental controls, security filtering, or quality-of-service rules. Temporarily disabling one feature at a time may reveal a conflict. Avoid leaving security protections off if the test does not clearly identify a cause.

Ask the provider to investigate a YouTube-specific routing or peering issue, not just whether the line is up. A connection can be fine for many services and still have trouble reaching a particular service. Mention that YouTube works over cellular and other Wi-Fi but fails across devices on this home connection.

If possible, record the time of a failure and run a speed or connection test then. A generic speed test may look normal, so describe the app-specific symptoms too. The provider may need to escalate it rather than just check the modem signal.

One quick way to separate router trouble from an upstream issue is to test YouTube through a reputable VPN for a short time, if you already have access to one. If it becomes normal through the VPN, that points toward a routing or filtering path difference, though it does not prove which party is responsible. If nothing changes, the test is less informative.

I would not jump straight to claims of intentional throttling. There are several possible network causes, and the pattern alone does not establish one.

Because this comes and goes, try collecting details while it is actually happening: whether wired devices are affected, whether the router shows errors or unusual traffic, and whether a router firmware update is available. If you change anything, make one change at a time. A factory reset can wipe settings, so save a configuration copy first if you go that route.