Gliders do tend to come down fast in rain. The wings are sanded, waxed, polished and what not to get a mirror shine, enabling the flow to remain laminar quite far back, usually all the way to the turbulator strips and those seem to be 2/3rds to 3/4ths of the cord back most of the time.
Rain and kamikaze bugs are naturally more detrimental to a wing with that finish than to a wing which has a more coarse surface finish by design.
Competition pilots fit their aircraft with bug wipers, which can somewhat clean off the leading edges in flight. In the hand computer based glider performance software, you can often enter a compensation factor for the amount of bugs on the leading edge to get accurate performance figures.
I had the interesting experience of flying underneath what must have been a mini-CB which decided to more or less explode in my face (or perhaps rather on top of my head?) a few months ago. Right when turning final, of course. I had waves forming across the top of the canopy, ASI going up and down by 30 km/h (with a Vref of about 105 that's significant) and VS oscillating between -3 and +2... not something I care much to repeat! Well, saved wear on the boards at least!
Cheers,
Fred