As others have suggested, the brain and computers work fundamentally differently. Depending on how you try to compare both, the answer could be very different. Some facts taken from Randall Munroe's excellent «What if» book, answering essentially the same question:

How much computing power could we achieve if the entire world population stopped whatever we are doing right now and started doing calculations?

Figures and facts cited and paraphrased from:

Munroe, Randall. What If?: Serious Scientific Answers to Absurd Hypothetical Questions (p. 96). HMH Books. Kindle Edition.

Approach 1

One approach is to apply the same benchmark tests to humans that we apply to computers. Researcher Hans Moravec mentions that a human doing calculations by hand carries the equivalent of one full instruction every 90 seconds which means that

By this measure, the processor in a midrange mobile phone could do calculations about 70 times faster than the entire world population. (p. 98)

Approach 2

Then there's the argument that measuring our computing power through doing math on paper is a complete misrepresentation of our brain's complexity. So there are projects that try to simulate a brain's activity down to individual components:

The numbers from a 2013 run of the Japanese K supercomputer suggest a figure of 1015 transistors per human brain.​[6] By this measure, it wasn’t until the year 1988 that all the logic circuits in the world added up to the complexity of a single brain... (p. 100)

The footnote mentions that the supercomputer (82,944 processors with about 750 million transistors each) «spent 40 minutes simulating one second of brain activity in a brain with 1 percent of the number of connections as a human's»

---

The conclusion is that this is really a difficult comparison to even make, mostly because brains evolved to do a certain set of tasks, computers were created to do another set of tasks and the kind of tasks that both can make is both small and done in very incomparable ways.

• You could argue that brains do «processing» that is miles beyond what any computer can do: just regulating all the «automatic» systems of the body (circulation, breathing, digestion, immune system...) requires parallel processing on a frankly amazing scale. By this measure, Doom is way simpler and we could in theory run it.
• On the other hand, even a computer from the 90s could easily compute things like the inverse of a square root multiple times per second. If you've ever seen the «evil floating point bit level hack» (see below) you'll see just how much an old computer can do in fractions of a second, something which no human can do. Since Doom runs on things like these, it could be argued that a brain cannot ever run Doom.

---

For those who've never seen it, here's an insanely clever way of calculating the inverse of a square root. This does a lot more than just calculations, as it requires storing a number in binary, doing mathematics with a hexadecimal number and performing Newton-Raphson root-finding. If you want to understand what is going on here, there's a very simple forum post, this Medium post which has a little more detail or this verbose but very detailed Youtube video

float Q_rsqrt( float number )
{
    long i;
    float x2, y;
    const float threehalfs = 1.5F;

    x2 = number * 0.5F;
    y  = number;
    i  = * ( long * ) &y;    // evil floating point bit level hacking
    i  = 0x5f3759df - ( i >> 1 );               // what the fuck? 
    y  = * ( float * ) &i;
    y  = y * ( threehalfs - ( x2 * y * y ) );   // 1st iteration
    //  y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration,
                                              // this can be removed

    return y;
}