Base64 implementation in C#
First and foremost: testing in Debug - Any CPU mode is bad. Debug configurations have a lot of extra overhead and Any CPU is a non-native instruction configuration*, both will cause you to get poor measurements.

Next, when you do your testing you should consider (I say consider because when doing simple comparisons you don't need to, but when trying to prove something you should definitely do so) using a proper benchmark tool like BenchmarkDotNet. (It's in NuGet so it's simple to install.)

However, if you choose not to (I'm not going to judge you for that) you should not measure the first execution of your code. You should loop it a few times (I usually use 10-128, depending on how fast the code is) and then discard those results, then do it again and keep those results. Take the average as your metric.

Why?

The first time you execute your code, the JITter (Just-In Time optimizer) will optimize your code another time. This adds a substantial amount of overhead to the first execution. Sometimes a lot of it. This can (and will) skew your results substantially.

* This is heavily debated and may or may not have any effect. In some testing in my environment x64 is faster, and in some testing Any CPU is faster.



Now that the lecture is over, let's look over the code quick:

You have a lot of "magic numbers" here, most notably 4 and 3, you should consider giving them a const identifier. (Consider what 4 and 3 mean in each instance, and what they mean if you were to be writing a Base32 converter instead of Base64.)

Your for loop (for (int i = 0; i &lt; div * 3; i += 3)) can use length instead of div * 3 there, just as well you should assign length first, then use it instead of data.Length everywhere. I.e.:

    int div = data.Length / 3;
    int mod = data.Length % 3;


Should instead be:

    int div = length / 3;
    int mod = length % 3;


With as frequently as you use dPointer[length - 1] and dPointer[length - 2], you should consider extracting variables there for extra micro-performance boosts.

Those are the only complaints I have, good work!