The Online Hate Prevention Institute's Andre Oboler, who has researched hate speech for more than a decade, said both platforms removed less than a quarter of posts flagged as being anti-Semitic.
Reddit deleted just 17 per cent, while X was slightly better at 24 per cent.
YouTube, LinkedIn and Telegram all removed just over 40 per cent.
TikTok - which had less media flagged than other platforms - deleted about 62 per cent, while Facebook removed 54 per cent.
But AI was presenting new challenges, Dr Oboler said, as users could find relatively easy ways to bypass hate-speech filters.
"What it's doing is allowing someone to upload a volume of new anti-Semitic content very rapidly," he told the royal commission into anti-Semitism.
Dr Oboler also accused the world's only trillionaire, Elon Musk, of helping to fuel anti-Semitism globally.
Mr Musk's X platform, formerly known as Twitter, had one of the highest rates of posts criticising Jews across social media, according to research from the institute.
"X was generally difficult to work with, particularly from Australia," Dr Oboler said.
He said Mr Musk's purchase of X in 2022 resulted in almost 80 per cent of trust and safety staff being fired immediately.
Since then, anti-Semitism on the platform had grown.
Mr Musk has previously denied accusations he is anti-Semitic or racist, claims levelled in 2023 after he posted in support of a conspiracy theory involving Jews.
Dr Oboler's team, which worked with a far-reaching definition of anti-Semitism that included anti-Zionism, identified a correlation between pro-Palestine activism and harmful rhetoric, although one didn't inherently lead to the other.
"I'm not saying that all those activities are anti-Semitic - far from it," Dr Oboler said.
"But there is a segment of anti-Semitism that goes on the bandwagon with that (activism) - and when there's less activism, there's therefore obviously less opportunity for that anti-Semitism to rise."
The third block of public hearings for the royal commission began on Monday with a focus on the treatment of Jewish people in mainstream and social media.
The ABC and SBS will appear later in the hearings, along with dozens of Jewish Australians who have been subject to online hate.
Experts in social media, online hate and extremism appeared throughout Wednesday.
Sydney University media analyst Terry Flew told the commission the re-election of US President Donald Trump had emboldened social media platforms to slash safeguards globally.
The US had condoned and fostered "a very strong alliance between many of the leading tech companies and that presidency", he said.
"(It has contributed to) the dismantling of a range of mechanisms that have been applied within the companies, such as the operation of trust and safety councils."
