It was fast before and works fine in my development environment.
There are many possible reasons why a site may be unexpectedly slow in production. The most common root causes are:
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Caching is not properly configured.
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The site is not able to scale to a higher level of traffic or quantity of data than seen in development.
Check the router cache
The first thing to check is that the router cache is configured correctly. On Platform.sh Professional the router includes a proxy cache out of the box but it must be configured correctly. The most common error is not whitelisting session cookies, which results in any page with any cookie at all never getting cached, even just analytics cookies. See the router cache documentation, specifically regarding the cookies property, for how to configure it properly.
For PHP projects there are also some additional tuning options available. See the PHP tuning guide for steps you can take to optimize your PHP configuration.
Further optimization tends to be very language and application specific. We strongly recommend using a good profiling tool to help locate particularly slow portions of your application. Generally just knowing where it is slow will suggest why and how to correct it.