In general: Servers get slower when there are more people on. We are uploading larger pictures, higher resolution, and more of them. 'Tis the season for the hobby so people are flocking to RC. Summer time people go back outside and the forums thin out and not many people go on so they clear up the bandwidth and things are much faster. Though you would hope with the amount that the sponsors/advertisers pay, RC is planning to get better/larger/faster servers. I'm sure RC has something up their sleeves to surprise us with soon enough.
IME-I don't have issues too often, just late at night around 11pm-1am is when I run into most issues.