My '03 Subaru Forester is at about 85k miles, and has been beautiful. I’ve recently noticed that when I try to accelerate quickly on the highway (eg: passing), the car stays the same speed for a second then “kicks” and speeds up. It feels like I’ve been rear-ended. I just got a free check-up on the car that saw cracked drive belts, dirty air filter, and slight head gasket leak; it also said “not a major concern at this time”. Is my acceleration issue related to one of these things, or is it something completely different? Thanks! 
Did you have the air filter replaced? Has it ever been replaced?
How about the fuel filter? I would start with those things, esp. the fuel filter.
And I’m not actually a mechanic, but I don’t think that any kind of head gasket leak, “slight” or not shouldn’t be seen as a major concern. Maybe given the way it is leaking it doesn’t need immediate attention - as in today - but it isn’t the kind of thing that I would just drive on with.
Certain years and models of Subaru are known for head gasket problems. I don’t know about the '03 Forester but there are several Subaru guys around who can provide info. Internet searches would also easily turn up info about it.