Cold starts can be one of the biggest trade offs when adopting a Serverless platform like Cloud Run, especially with 30 second start up times. We introduced minimum instances[1], currently in beta, which will allow you to specify a minimum number of container instances to be kept warm and ready to serve requests.
There is a cost for doing so as instances kept running in this way do incur billing costs [1].
I've done some work with Ruby in the past, and based on my experience, you might not be able to take advantage of Cloud Run's best feature, concurrency[2], which is another way to reduce cold starts by routing concurrent requests to the same container instance. Ruby maybe blocking in flight requests and forcing us to fire up another instance to handle it. Once the first request has completed there is a chance the container instance processing that request will be spun down, this is something minimum instances can help with. You might need more than one minimum instance to compensate for the lack of concurrency if you really want to see improvement in overall latency.
[1] https://cloud.google.com/run/docs/configuring/min-instances
[2] https://cloud.google.com/run/docs/about-concurrency