Q:

Performance Impact of DescribeSObjects vs. Individual Describe Calls?

I've been wondering this for a while, so it's time to put this question out there. How does Schema.describeSObjects perform when compared to individual calls to SObjectType.getDescribe?
For example, which of these is faster? By how much?
static Map<String, SObjectType> globalDescribe = Schema.getGlobalDescribe();
public static List<DescribeSObjectResult> getIndividualDescribes(List<String> types)
{
    List<DescribeSObjectResult> describes = new List<DescribeSObjectResult>();
    for (String sObjectType : types)
        describes.add(globalDescribe.get(sObjectType).getDescribe());
    return describes;
}
public static List<DescribeSObjectResult> getBulkDescribes(List<String> types)
{
    return Schema.describeSObjects(types);
}

A:

Getting the individual describes is significantly faster. Depending on your situation, it is anywhere from 14 to 850 times as fast!
Describing Many Objects Once
The first continuum I profiled is the situation where you have many objects to describe and make one call to describe all of them.
The individual getDescribe approach can describe anywhere from 5 to 11 objects per millisecond, whereas the bulk describeSObjects approach takes anywhere from 2 to 3.5 milliseconds per object. In this continuum, individual describes perform roughly 14 to 30 times better than the bulk describe method.
Here is the amount of CPU time consumed for each approach to describe a given number of sObjects once:
Objects  |  Individual  |    Bulk  |  Multiplier
================================================
      5  |         1.1  |    17.5  |        15.9
     10  |         1.5  |    24.6  |        16.4
     15  |         2.1  |    29.7  |        14.1
     20  |         2.6  |    38.7  |        14.9
     25  |         2.8  |    71.4  |        25.5
     30  |         3.1  |    88.7  |        28.6
     35  |         3.3  |   107.7  |        32.6
     40  |         4.6  |   117.2  |        25.5
     45  |         4.3  |   133.4  |        31.0
     50  |         4.7  |   144.3  |        30.7
     55  |         5.2  |   151.4  |        29.1
     60  |         5.9  |   153.7  |        26.1
     65  |         6.2  |   156.5  |        25.2
     70  |         6.8  |   167.3  |        24.6
     75  |         7.3  |   174.7  |        23.9
     80  |         7.6  |   184.7  |        24.3
     85  |         7.9  |   192.4  |        24.4
     90  |         9.0  |   193.0  |        21.4
     95  |         8.7  |   198.1  |        22.8
    100  |         9.1  |   209.7  |        23.0

Describing One Object Many Times
The second continuum I profiled is the situation where you describe one object many times, for instance if you have describes in a loop somewhere in your code.
The individual getDescribe approach can describe an object anywhere from 17 to 33 times per second, whereas the bulk describeSObjects approach takes anywhere from 25 to 29 milliseconds per describe. In this continuum, individual describes perform hundreds of times better than the bulk describe method.
Here is the amount of CPU time consumed for each approach to describe one object (User) a given number of times:
Iterations  |  Individual  |     Bulk  |  Multiplier
====================================================
        25  |         1.5  |    719.7  |       479.8
        50  |         2.6  |   1408.4  |       541.7
        75  |         2.9  |   2147.9  |       740.7
       100  |         4.3  |   2782.4  |       647.1
       125  |         4.4  |   3667.9  |       833.6
       150  |         5.2  |   4171.3  |       802.2
       175  |         6.1  |   4640.4  |       760.7
       200  |         7.3  |   5167.1  |       707.8
       225  |         7.2  |   5887.2  |       817.7
       250  |         7.8  |   6618.9  |       848.6
       275  |         8.7  |   7175.3  |       824.7
       300  |         9.0  |   7537.6  |       837.5
       325  |         9.8  |   8328.8  |       849.9
       350  |        11.1  |   9090.1  |       818.9
       375  |        12.9  |   9423.5  |       730.5
       400  |        12.3  |  10422.8  |       847.4
       425  |        13.5  |  10716.1  |       793.8
       450  |        13.6  |  11431.6  |       840.6
       475  |        14.5  |  12063.8  |       832.0
       500  |        14.8  |  12977.7  |       876.9

