Joint Venture Silicon Valley's annual economic study found that 2015 was a year of record-breaking growth and prosperity, but it cautioned that there are perils associated with this kind of break-neck trajectory: A fourth-quarter slowdown could portend the beginning of the burst of a bubble.

The 2016 Silicon Valley Index, a regional study published since 1995, found that employment increased 4.3 percent in 2015, a level unprecedented in any other year except 2000, Russell Hancock, president and CEO of Joint Venture, said during a Wednesday press conference.

The Bay Area created 129,223 new jobs between the second quarter of 2014 and the second quarter of 2015. Job growth has been accelerating since 2010, with the most rapid growth during that time period.

"It's like adding two cities the size of Santa Clara to our region," he said.

What's driving it? Tech, Hancock said. More than 37 percent of job growth was in the tech category of innovation and information, which includes products and services, computer software, apps, devices, the cloud and the like.

The next biggest growth occurred in community infrastructure: health care, construction and education -- areas that support the tech growth, he said. A full 50 percent of the job growth was in this category, with tech jobs accounting for 25 percent, he said.

Another large-growth area: accommodations and food service, including restaurants, hotels and catering. In 2015, the area had the largest amount of planned and completed hotel development.

Biotech is a small regional employer, but it accounted for 20 percent of the growth, garnering the largest share of IPOs over the last three years, according to the Index.

Jobs in the sector of storage and warehousing (of tech furniture and electronic components) increased by 22 percent.

Few categories had job losses -- only eight out of 46 categories.

"That has never happened," Hancock said. "Last year (2014) we had losses in half of the categories."

The declines occurred in telecommunications manufacturing, semiconductor and equipment manufacturing, IT repair services, nonprofit organizations, and food and beverage manufacturing. The prosperity also did not translate to government.

In innovation and entrepreneurship, the number of patents were the highest ever recorded -- 19,414 -- a 14 percent growth over 2014. Six Silicon Valley cities were among the top 10 in the U.S. for patent filings, Hancock said. Most of those were in computers, data processing and information storage.

Venture capitalists also invested $24.5 billion. Silicon Valley garnered $11.1 billion of that money, but San Francisco outstripped the area -- with $13.3 billion -- for the second year in row. Silicon Valley residents still outstrip San Francisco and all other areas in income gains. The average annual earning in 2015 was $122,000, with San Francisco earners making $111,000 and the Bay Area overall average at $99,000.

But the phenomenal prosperity has some perils, the Index found.

"Growth income is not evenly distributed," Hancock noted. High-wage jobs -- more than $125,000 annually -- account for 25 percent of the .

"Thirty-two percent are low wage, making less than $30,000, which isn't even a living in Silicon Valley," he said.

Forty-three percent of jobs are in the mid-range professions, which are shrinking by 1 percent per year, he said.

"Low-wage earners' wages are stagnating," he added.

Three out of 10 people are not earning enough to meet California Self-Sufficiency Standards, which include food, rent and other basic necessities.

Who gets a slice of the housing pie, or for that matter, the rental housing, is also shrinking.

"The people who are moving into Silicon Valley are those who can afford it," Hancock said, noting that largely comprises investors and moneyed persons rather than any other group. The houses being built and sold are largely luxury homes with little growth of affordable housing.

The number of homes sold is down. Inventory has declined more than 60 percent since 2011. The median sale price in San Mateo County is $926,000. In Santa Clara County, it's $830,000.

Just 27 percent of first-time home buyers can afford a median-price home in San Mateo County; 41 percent in Santa Clara County.

But the rate of residents who are "burdened," meaning they are spending more than 35 percent of their income on housing, is 39 percent for homeowners and 39 percent of renters. The average Silicon Valley rent is $2,749 per month; the average for condos and homeowners is $3,500 per month. Between 2011 and 2015, the average rent rose 33 percent.

Silicon Valley is not doing well at building housing, Hancock said. In 2015, only 5,055 residences were approved for permits compared to 11,000 in 2014, he said.

Whether the rapid growth will continue remains what Hancock called the "gorilla in the room." The fourth quarter of 2015 saw a slowdown. In Silicon Valley, venture capital investment saw a 31 percent drop; in San Francisco, the decline was 50 percent.

And IPOs have also slowed. In 2015, there were 16 in Silicon Valley and six out of San Francisco. That's a drop from 2014, when there were 23 in Silicon Valley.

"Three quarters of the IPOs were before late August, before the big stock market drop," Hancock said. In January 2016, there were no IPOs. It was the first IPO-free month since 2011."

The full report can be found on the Joint Venture site at jointventure.org.