I just needed the "first word" achievement
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#18
"Is being a tempo down losing?"
++ No.
Being down a pawn without compensation is generally losing.
3 tempi = 1 pawn
Hence black can afford to lose 1 tempo and white can afford to lose 2 tempi.
For example 1 e4 d5 2 exd5 Qxd5 3 Nc3 Qd8 is viable and has been played many times by top grandmasters including Carlsen. The lost tempo does no harm. Black lost 2 tempi 2...Qxd5 and 3...Qd8, but white lost 1 tempo 2 exd5, so black had a net loss of 2 - 1 = 1 tempo.
In your example black loses 2 tempi: first 3...a5 which contributes nothing to either the center or development of pieces into play and is thus a lost move and second your Bc5 will be chased after 4 c3 and 5 d4.
The reason why black is then lost is not only the 2 lost tempi, but also that white after 4 c3 and 5 d4 gains full control over the center.
White delivering checkmate may be 30 moves away, but is inevitable if both sides play their best available moves.
Compare with 1 e4 e5 2 Nf3 Nc6 3 Bc4 Bc5 4 b4 Bxb4 5 c3 Bc5 6 d4. White also gains tempi and central control, but black has a full pawn as compensation. In some variations black gives back the pawn te regain tempi and get grip over the center.
I don't understand the plan with 3. - a5
White can continue with 4. c3 and if 4. - Nf6 5. Ng5!
How is this bad?
It's similar to two knights defence with a tempo up with c3 for white since a5 for black is useless in this line. Can't be bad for white then. f7 is weak point.
Is being a tempo down losing?
No, but it's a disadvantage.
Is there a way to make the disadvantage worse?
What do you mean? A disadvantage is a disadvantage. How it will fall out depends on how people are playing.