On reddit /wallstreetbets people report other trading platforms down as well, not sure if true
Typically, Robinhood is always down at market open (even on low volume days).
On high volume days, IB has been down several times.
However, the thinkorswim platform on TD is great and I've yet to encounter downtime.
Regarding TD, https://twitter.com/TDAmeritrade/status/1354447688205443076