A surge in share trading activity on August 31 and September 1, which included an uptick in stocks like Apple and Tesla, has led to delays on the popular trading app Robinhood, and a flurry of customer complaints on social media.

On September 1 morning, Robinhood users encountered messages like the one below, which noted the app has been experiencing an uptick in usage that had affected its performance.

The notices appeared a day after Robinhood issued an apology on Twitter to customers who had experienced delays in placing their orders.

This morning, some customers were experiencing issues with delayed order status updates. Wea€?ve resolved the issue and Robinhood is operational. We apologize and thank you for your patience.

—Robinhood Help (@AskRobinhood) August 31, 2020

The slowdowns this week appear relatively minor compared to those that occurred in March when a massive surge in trading volume led Robinhood to crash for long stretches, including one outage that lasted an entire day.

Customers nonetheless took to Twitter to vent about the latest delays, and mock Robinhood's performance.

Robinhood users trying to get their app working (when the whole system is dead in the water right now) pic.twitter.com/wtAQ80A0LV

—Robert (@outlaw2097) September 1, 2020

A person familiar with Robinhood told Fortune that "Some of our customers were briefly experiencing degraded performance on our web app" but did not provide additional details.

Numerous Robinhood customers on Twitter cited delays arising from their attempts to purchase shares of Tesla and Apple, which respectively carried out 5-1 and 4-1 stock splits on August 31 that led to a wave of purchases.

Over 225 million shares of Apple changed hands on August 31, according to data from Bloomberg, which represents a more than fivefold increase from August 28. In the case of Tesla, August 31's figure was nearly 120 million shares compared to around 20 million August 27 and August 28.

Robinhood was not the only brokerage affected by the surge in trading, as on August 31 Fox Business reported outages at TD Ameritrade and Charles Schwab.

The latest service disruptions come at an awkward time for Robinhood. While the company has enjoyed huge customer and revenue growth in 2020, and raised large amounts of money from investors, it is also facing scrutiny over its customer service and technical stability.

Robinhood is facing class action lawsuits over the outages in March from customers who allege the incident caused them to lose money. And a new report noted that regulators received nearly 500 complaints about Robinhood this year, more than its brokerage competitors combined.

The controversies come as Robinhood is adding senior executives specializing in compliance ahead of rumors the company soon plans to go public.