Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some fast-growing stocks to your portfolio but don't have the time or expertise to hand-pick a few, the Guggenheim S&P Mid Cap 400 Pure Growth ETF (NYSEMKT: RFG) could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of them simultaneously. It focuses on mid-sized companies with growth characteristics, such as sales and earnings growth and momentum.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The Guggenheim ETF's expense ratio -- its annual fee -- is a rather low 0.35%.

This ETF has performed well, trouncing the S&P 500 over the past five years, and topping it handily over the past three. As with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

Why pure-growth mid caps?
Mid-cap stocks make a lot of sense for our portfolios, because they're somewhat proven, having grown to mid-cap size. Better still, they tend to still have plenty of room for further growth – and the ones in this ETF have demonstrated some brisk growth.

More than a handful of "pure growth" companies had strong performances over the past year. 3D Systems (NYSE: DDD) soared 97%, nearly doubling. The company just reported its second-quarter earnings, with revenue up 45% and net income up 12%, though EPS shrank a bit, because of a big jump in the number of shares outstanding. 3-D printing is still in its infancy, with much promise. 3D Systems has been growing in part via acquisitions, and has been boosting its investments in R&D as it launches new products. Some see these shares as a bit rich now, though, and there has been insider selling.

Georgia-based Synovus Financial (NYSE: SNV) surged 76%, and a handful of insiders have been buying shares recently. The company recently repaid its $968 million TARP debt, in part by issuing more stock. Its credit quality has been improving, and it has been posting strong return-on-equity numbers. It just reported its fourth profitable quarter in a row, too.

Vertex Pharmaceuticals (NASDAQ: VRTX) jumped 59%, and just reported solid earnings on strong sales of its cystic fibrosis drug, Kalydeco. (It's looking to expand approvals for the drug, too.) A Vertex hepatitis C drug has not met with such success, though. Vertex's history offers valuable lessons to investors, such as the importance of patience and how a company's prospects can change quickly.

Oil refiner HollyFrontier (NYSE: HFC) advanced 32%, and yields 2.7%. It was up by more earlier in the year, but has been hurt by a contraction in the difference between prices of West Texas Intermediate crude oil and that of Brent crude, as well as some operational issues. Like some peers, the company is using rail to transport oil, and some see the stock as undervalued.

The big picture
A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.

Longtime Fool contributor Selena Maranjianwhom you can follow on Twitter, has no position in any stocks mentioned. The Motley Fool recommends 3D Systems and Vertex Pharmaceuticals, owns shares of 3D Systems, and has options on 3D Systems. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.