Does HTTPS Slow Down a Site?
The short answer: a small cost that can be reduced
Does HTTPS slow down website loading? Slightly, at the start of each new connection, but modern setups keep the difference small. TLS adds work during the handshake and some processing for every encrypted byte, and both costs can be reduced with current settings.
The overhead falls into two categories. The first is latency, the extra round trips needed before data can flow. The second is computation, the work of performing key exchange and encrypting data. Visitors rarely feel the second, because modern processors handle encryption efficiently.
Where the time actually goes
Latency is usually the larger factor. A new connection under TLS 1.2 adds about two round trips before the first request can be sent. Each round trip can take tens or hundreds of milliseconds on long or mobile links.
TLS 1.3 needs one round trip for a new connection, which cuts that delay. Resumed connections can go further and let a returning visitor send data sooner. The handshake is where these round trips happen.
Computation is a smaller cost. The asymmetric operations used in the handshake take some CPU time, but a server handles many of them every second. Bulk encryption of page content is cheap on current hardware, especially when the processor has dedicated instructions for it.
Settings that reduce the cost
Several features reduce TLS overhead. Using the newer protocol version where clients support it shortens the handshake. Session resumption lets returning visitors skip the full key exchange, which saves both time and computation.
OCSP stapling helps too. Without it, a browser may fetch revocation status from a separate server during the first visit, which adds another round trip. With stapling, the server supplies that status along with its certificate.
Where a content delivery network terminates TLS close to the visitor, the handshake round trips cover a shorter distance. That is one reason many sites place a CDN in front of their origin, and the termination modes then decide how the rest of the path is protected.
The same site, two different visitors
Consider two visitors to the same news site. One is in the same city as the server and the other is on a mobile connection in another country. For the nearby visitor, the handshake takes a few milliseconds and is barely noticeable. For the distant visitor, each round trip is far longer, so the choice of TLS version and resumption has a much bigger effect on how fast the first page starts.
Frequently asked questions
Is HTTPS slower than HTTP?
Every new HTTPS connection takes a little longer to set up than plain HTTP, because of the TLS handshake. After that, the difference in data transfer is small. Well-configured HTTPS sites often load about as fast as HTTP ones, and HTTPS is needed for many modern browser features.
Does the size of a certificate affect page speed?
It can. The certificate and any intermediate certificates are sent during the handshake, so larger chains take more bytes and more time on slow links. Keeping the chain to what clients actually need keeps this cost small.