I've flicked through the source code of the most common reflector constructors, and I see nothing that is obviously causing such problems.

Can you provide some example code that behaves extra ordinarily, perhaps that will help me to understand better ?

Literally any reflection object, krakjoe.

It is comparable in performance to the creation of other objects, sure, but it is too slow considered that reflection is basically just static information.

For instance, why is this 6 (!!!) times slower than creating the actual object?

class A{}

for($i = 0; $i < 1000000; $i++){
    new A();
} // 0.3150179386 seconds

for($i = 0; $i < 1000000; $i++){
    new ReflectionClass('A');
} // 1.9061090946 seconds

The example would probably make more sense if I assigned the object to a variable. 

Also notice how I left out the constructor. I'd expect ReflectionClass to compare in performance to the creation of an object without calling its constructor.

Sorry for the multiple replies.

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The only obvious inefficiency in the ReflectionClass constructor is the population of the "name" property -- which will allocate and then destroy a temporary string. If we replace this with a persistent string we might get a 10% improvement there.

The difference between "new A" and "new ReflectionClass('A')" is that the latter has to perform a constructor call (expensive) and a runtime lookup of the class 'A' (expensive). This already accounts for most of the difference.

Some ctors like ReflectionProperty::__construct() could be optimized by using a union for different reflection object structures, rather than a separate allocation.

Based on the script provided by OP:

<?php
class A{}

$t = microtime(true);
for($i = 0; $i < 50000000; $i++){
    new A();
}
var_dump(microtime(true) - $t);

$t = microtime(true);
for($i = 0; $i < 50000000; $i++){
    new ReflectionClass('A');
}
var_dump(microtime(true) - $t);

Numbers on master:

float(1.1946558952332)
float(3.4293010234833)

So ReflectionClass construction is now a bit less than 3x slower than the object construction.

Today, the reflection class constructor makes no duplicate strings. You still have to perform a lookup if you pass the class name as a string, which as mentioned accounts for the majority of the difference. Comparing new Class to new ReflectionClass doesn't make a lot of sense, new has a cache slot for the class entry.

I think there's not much more that can be done here, so I'm closing this bug.