I think that's where people tend to not understand lightroom when they compare  it to Photoshop.
I love photoshop and I love lightroom. doing photos i wouldnt pick one or the other I find you really need them both. light room is perfect for having better control over colors and levels and exposure adjustments. I find they are not even close to levels and exposure that photoshop has. the ones in light room seem so much more advance and a lot more control.

and photoshop is great for going in and fixing flaws in a photo like a pimple on a model bring out the highlights in her hair and so on.

lightroom you really have to look at it as a tool to get the raw photo perfect... for example in a photo you tend to get hot spots or way to dark of blacks. lets say you shot a white sheet with great patterns. but you have a hotspot that kills that pattern. I have never been able to bring that back in photoshop. but in light room slide the recovery fader and the hotspot falls out and im amazed the pattern hidden was there all along. this photoshop will not do. and dark spots use the fill light fader and you find neat things in the shadow without a ton of pixel problems that photoshop would give you.

so my view on the 2 is you can not compare the 2 its apples and oranges . if I didnt have both after doing a shoot I wouldnt even try an edit any of my photos lol

and to end the computer slowing down when using light room. dont allow it to open up every picture you have at once. just open the ones you want to work on. and the slow down problem goes away. lightroom wants to import every image in any folder you choose and that causes a slow down. you can change this and fix that problem as well :-)

it also creates a back up on your main C drive of everything you do. make sure you shut that off as well because after a month you will max out your C drive and that will cause a slowdown as well

hope this helps