Having just migrated to Rackspace, and realizing that ASP.NET is a pain in a cloud, due to security levels, I think it is still going to be a better choice, as you can quickly scale due to rising demand more quickly than trying to buy and install the computers yourself, but, as others mentioned there are design considerations to take into effect.
For example, can you take advantage of the memory and use memcache, http://php.net/manual/en/book.memcache.php, to help you with speed?
Profiling is still important, to know where the bottlenecks are.
For example, do you spend a great deal of time in the database? If that is going to be a problem, then start to look at what solutions the various cloud vendors offer to help with that.
I think, ultimately, you are better in a cloud, if their increased security isn't going to be a problem for you.
But, if you are really concerned about speed, you may need to look at moving away from PHP at some point, as, there will be a point where changing to a compiled language, but that depends on what you are doing, as to whether you would see any improvement there. I haven't tried it, but you could try compiling PHP, http://www.phpcompiler.org/. The best language for any problem depends on what your needs are and what you are doing.