How much does rain slow you down

A lot! No data here, however I slow down considerably going into any corners/curves. In a 2009 HIM I went around a curve (when it was raining) at 27MPH, normally I would have been fine but I just kept sliding towards the edge of the road until I reached the edge. At which point I went into my best impersonation of superman, fortunately it was all grass so injuries were limited to a bloody lip and a cracked helmet.

On straight roads with light rain, I think it slows me up a little but don't know if its mental or actual physical parameters such as rolling resistance.

Off the top of my head, air density increases correct. I know there are some calculators on-line(somewhere) using this as part of they power calcs. Should be able to estimate air density on dry days vs raining.

Rolling resistance will increase as well but by how much--??? Don't know of any testing on this, however since the tires pick water up off the ground and spray it(on your back) that requires energy that is not required on a dry day.

It depends on how hard it is raining and what the road conditions are. I also raced in the rain on Sunday. There weren't many hills to speak of, but there were a lot of sharp corners and two traffic circles. I averaged about 4 mph slower than I expected to, but I just wasn't comfortable going faster, especially since I don't have much history riding aero and that bike is a bit twitchy.

I raced in the rain yesterday too (Devilman). The cold was a whole other issue, but the rain itself also impacted my bike. In addition to the other items already noted (rolling resistance, slower in turns, denser air) I also wore allot of clothing (including non-aero jacket) and carried significantly more weight (everything I wore was completely soaked through). I did not have power readings since I took my road bike with aerobars instead of my less stable tri bike, but I was 3 mph slower than planned and consider that a victory.