I know you have to practice to get it down but how do you actually comprehend what notes a guitarist is playing when he is going fast? There is a lot of songs I want to learn but I can't because I can't figure out how to play the tabs correctly. Some tabs I find are weird and completely off so that won't help me.
fast passages usually sound strange at slow tempo's so you might have to start with an easier solo before you advance to one that you cant comprehend.

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metronome. Slower tempo. One measure at a time.
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What SPBY said. You might want to get more practice and get more used to hearing it before trying to learn it. In time it'll begin to make more and more sense.
if you use power tab or guitar pro, you can slow down the solo and get the timing right at a slower tempo
I don't think he means how to play that fast, but actually get each accurate note for yourself instead of following in inaccurate TAB.

Its a difficult process because when something so fast in terms in of music is slowed down, it sounds weird and completely different due to gain/distortion, hammer on/pull off's.

Your best bet it to find each and every single note by slowing something down and recording each note, it is a very tedious process though. :/
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get guitar pro...and you can highlight every note on the tab and see exactly what it is...so if they are going back between like 3 different notes you can see what they are doing..and you can change it if u feel it is slightly off! slow it down too...its the best...get it...i would say a must for anyone that uses online tabs