A systemic review of 26 studies [1] in 2019 found that:

"Although variability in the cannabis products used, outcomes assessed, and study quality limits the conclusions that can be made, modest reductions in cognitive performance were generally detected with higher doses and heavier lifetime use."

The American Journal of Psychiatry [2]:

"Long-term cannabis users showed cognitive deficits and smaller hippocampal volume in midlife. Research is needed to ascertain whether long-term cannabis users show elevated rates of dementia in later life."

This study published in JAMA [3]:

"...among 3385 participants with cognitive function measurements at the year 25 visit, 2852 (84.3%) reported past marijuana use, but only 392 (11.6%) continued to use marijuana into middle age. Current use of marijuana was associated with worse verbal memory and processing speed; cumulative lifetime exposure was associated with worse performance in all 3 domains of cognitive function. After excluding current users and adjusting for potential confounders, cumulative lifetime exposure to marijuana remained significantly associated with worse verbal memory..."

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7259587/

[2] https://ajp.psychiatryonline.org/doi/full/10.1176/appi.ajp.2...

[3] https://jamanetwork.com/journals/jamainternalmedicine/fullar...