The volcano in Iceland has been playing havoc with Eurocontrol and getting slot times, sometimes 2 hours after scheduled departure or being canceled outright. Depending on where the ash is, sometimes they've been down to only two NAT Tracks, and those go up as far as 66N, increasing flight time. I'm afraid it'll be like this all summer.

If traveling to Europe this summer, bring along a lot of patience, and give yourself plenty of wiggle-room on being someplace.