"A webpage is slowing down your browser.." when accessing ProtonMail with Tor Browser 7.0-build1 (this did not happen in TBB 6.5)

"A webpage is slowing down your browser.." box shows up when log in into ProtonMail (with their onion service https://protonirockerxow.onion/) with Tor Browser 7.0a4 and 7.0-build1 (this did not happen in TBB 6.5)

Security Slider set to Medium in all cases.

This did not happen for me with TBB 6.5.

Wonder if it may be a defect or something normal.