We can use the Data Explorer to see how often we up- and downvote. This query says a post receives (on average) 5.29 upvotes and 0.46 downvotes.
With the 'site switcher' you can compare the figures to other sites in the network, e.g. Stack Overflow which has 2.24 upvotes and 0.29 downvotes. Unfortunately, an attempt to make a table for all sites in the network times out.
We can't use SEDE to measure how fast we downvote; see Why is vote time missing in the SE data dump and SEDE? for details.