There is a question this page is not answering: whether a VPN costs speed in general. It does, a little, and we measured how much and why.
This page answers the other one, yours is slow right now, and you want to know which of seven things is doing it. (If it is not slow but keeps dropping, that is a different list entirely: start with the question that splits the disconnection causes in two.) They are ordered below by how quickly each one can be ruled out, not by how common they are, because the fastest diagnosis is the one that eliminates the most possibilities per test.
Step 0, Is the VPN even the problem? (two minutes)
Before anything else, get two numbers. Run a speed test with the VPN off, write the figure down, then run the same test with the VPN on. Same test, same moment.
If the two numbers are close, stop here: your connection was already slow and the VPN is not the cause. This is the single most common outcome, and it is why the rest of this page is worth nothing until you have those two figures. Congested Wi-Fi, a saturated uplink or an evening peak on your line will all look like "the VPN is slow" to someone who only tested with it on.
If the drop is large, half your speed or worse, continue.
Step 1, Change server, same country
Your setup did not change since yesterday, so a sudden slowdown points at the far end. Popular servers get crowded, and a server that was fast last week can be oversubscribed today.
Switch to a different server in the same country and re-test. This one test separates a server problem from a client problem, and it takes thirty seconds. Do it before touching any setting.
Step 2, Move closer
If several servers in that country are slow, try one geographically nearer to you.
Distance is not negotiable: light in fibre covers roughly 200 000 km per second, so a round trip to another continent costs tens of milliseconds whatever the provider claims. Latency hurts responsiveness directly, and throughput indirectly, a long path takes longer to reach full speed. For most people this is the highest-value single change.
Step 3, Change protocol
If your client offers a choice and you are on OpenVPN, switch to WireGuard and re-test.
WireGuard runs in kernel space with a small fixed cipher set; OpenVPN runs in userland with a configurable one. The difference is structural, and it usually shows. It is not a guarantee, a congested server is slow on any protocol, but it is one click, so it belongs early in the ladder.
Step 4, Test on a cable, or on mobile data
If the VPN is slow on Wi-Fi, the wireless link is a suspect in its own right. Channel congestion, distance from the router or an ageing 2.4 GHz connection all reduce headroom, and the VPN's overhead then makes an existing weakness visible.
Testing the same server over Ethernet, or over mobile data, tells you which side the ceiling is on.
Step 5, Look for throttling
A distinctive pattern: your normal connection is fast, the VPN is slow on every server you try, and the slowdown is consistent rather than variable.
Some networks deprioritise recognisable VPN traffic. Trying an obfuscated mode or a different port is both the remedy and the test, if obfuscation restores your speed, throttling was the cause. This is the same mechanism used to get through national filtering, covered in our VPN obfuscation guide.
Step 6, Check MTU, if the symptom is odd
The signature here is unusual: the connection works, small requests are fine, but large transfers stall or web pages hang half-loaded. That is a fragmentation problem rather than a bandwidth one, and lowering the MTU on the client fixes it.
If your slowdown is uniform across everything, this is not your cause, skip it.
Step 7, Check what else is using the line
Last because it is the least likely to be forgotten and the easiest to see: another device streaming, a backup running, a game updating. The VPN did not slow down; the share of the line left for it did.
The point of the order
Each step above either eliminates a whole family of causes or points at one. Step 0 alone resolves most cases, and it is the one people skip, they change servers, reinstall the client and try three protocols before discovering their connection was slow to begin with.
If you reach the end with the drop unexplained and reproducible on several servers, the honest conclusion is that the provider's capacity on your route is the limit. That is a reason to test another provider, not another setting.
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