It appears to me that rackspace somehow freezes cloud VMs after some idle time. This means the first page request to a php page takes much longer to respond than the subsequent requests.
I am actually querying a machine with wget from a different host now to keep it "alive". But I wonder what frequency would be necessary. Does anyone know the time period after which they send a VM to "sleep"? I guess it would be some minutes.
EDIT: There is no caching involved on the php site.
It just recently moved from another vhost and there was never such latency on the first request.