No, a slower lens (higher f-number, or, actually, T-number) is safer than a faster lens. It's only a practical accident that a long "telephoto" lens is usually significantly slower than a lens closer to normal for the format (and that the long end of a consumer zoom is usually slower than the wide end). A 600mm f/4 lens is an eye-wateringly expensive hulking monster; a 50mm f/1.8 lens is the proverbial cheap-as-chips (with some exceptions), but will let about 5-6 times the amount of light through to be imaged on the sensor - it'll burn a smaller hole, but do it much more quickly. Should you be able too find a 600mm lens with the same T-number (similar to the f-number, but taking transmissivity losses into account), it'll do the job just as quickly, but much more thoroughly.