Things are looking pretty good for activist investor Bill Ackman.

The net asset value of Pershing Square Holdings, Ackman's publicly traded vehicle, rose 36.9% in the first quarter of 2019 and remains up more than 38% so far this year, according to a shareholder letter released Friday. By comparison, the S&P 500 is up 14.7% and has returned 15.6% this year.

The 2019 gains may help attract flows back into Ackman's fund, which suffered a string of losses in recent years. In early 2018, the fund's assets were down by roughly half from their $20 billion peak in 2015. Pershing Square's net asset value dropped by 4% in 2017 while the rose nearly 20% that year.

Pershing's 2019 success is in large part thanks to the outperformance of a handful of Ackman's stock picks. He manages the public vehicle's portfolio though investment firm Pershing Square Capital Management.

In his letter, Ackman highlighted "significant progress" at burrito chain Chipotle Mexican Grill under CEO Brian Niccol, noting same-store sales and transaction growth of 10% and 6%, respectively, in the first quarter.

"Despite the 63% year-to-date increase in the share price, we believe that Chipotle is in the early innings of its transformation," he wrote to stakeholders.

"Key drivers for continued same-store sales progress in the coming quarters include the new loyalty program launched in March, the completion of the rollout of the digitized second make line by the end of 2019 from half to all stores, and a continued focus on improving speed of service and store employee retention," he added.

A second make line is a feature Chipotle is adding to its kitchens to increase productivity, specifically for digital orders.

Pershing and other shareholders applauded Niccol's appointment to chief executive officer in 2018 after a successful tenure at Taco Bell. For Ackman — long a proponent of menu and service innovations at his restaurant investments — Niccol's hand in Taco Bell's "Live Mas" tagline and popular Doritos Locos Tacos likely represented a chance to bring similar ingenuity to Chipotle.

Ackman first bought Chipotle stock in 2016 at an average price of $405 per share. Since then, the stock has gained 76% to about $713 Friday.

Restaurant Brands International is also well ahead of the S&P 500 this year, buoyed by organic pretax earnings growth at each of its three brands. Canadian coffee chain Tim Hortons in particular should be set for big sales growth following the launch of its loyalty program less than one month ago; nearly 20% of Canadians have already enrolled, Ackman wrote.

To compare, about 5% of the U.S. population is enrolled in the Starbucks Rewards loyalty program. Pershing Square is also an investor in the Seattle-based coffee company.