NB: This is not a duplicate of this issue as I am experiencing a significant performance drop with all HD packs, 32x and up.
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I am the author of Defscape, a 16x, 32x and 64x texture pack.

Having used 1.5 for quite some time while texturing, I've noticed that native HD texture performance is very poor.

Changing settings doesn't seem to affect my FPS, although V-sync on tends to lower my frames.

The following results use the settings:

Distance: Normal
Graphics: Fancy
Smooth lighting: On/Maximum (there is no difference in performance between minimum/maximum)

13w12~

Default: 40-50 FPS (70 FPS looking at the sky - 75 FPS looking at the ground)
16x: 35-50 FPS (60 FPS looking at the sky - 65 FPS looking at the ground)
32x: 25-45 FPS (45 FPS looking at the sky - 50 FPS looking at the ground)
64x: 10-20 FPS (20 FPS looking at the sky - 30 FPS looking at the ground)

Note: Although slightly smoother, frames are generally stuttery and unsmooth, no matter the figure in f3.

1.5:

Default: 25-40 FPS (120 FPS looking at the sky)
16x: 20-35 FPS (70 FPS looking at the sky)
32x: 15-30 FPS (50 FPS looking at the sky)
64x: 10-20 FPS (20 FPS looking at the sky)

Note: All frames are generally stuttery and unsmooth, no matter the figure in f3.

1.4.7:

Default: 35-60 FPS (200 FPS looking at the sky)
16x: 35-60 FPS (200 FPS looking at the sky)
32x: 35-60 FPS (180 FPS looking at the sky)
64x: 35-60 FPS (180 FPS looking at the sky)

Note: All frames are smooth.

1.5 & Optifine:

Default: 60-100 FPS (240 FPS looking at the sky - 120 FPS looking at the ground)
16x: 60-100 FPS (220 FPS looking at the sky - 120 FPS looking at the ground)
32x: 60-100 FPS (210 FPS looking at the sky - 115 FPS looking at the ground)
64x: 60-100 FPS (210 FPS looking at the sky - 115 FPS looking at the ground)

At the moment, the only way 1.5 is playable for me is by using Optifine which seems to fix all the lag-enducing issues that have been introduced between 1.4.7 and 1.5.

That said, having to use a third party modification obviously isn't ideal and shouldn't be a prerequisite for smooth choppy-less gameplay with HD texture packs.

If you wish to test this yourself, here are the different Defscape resolutions:

Mediafire:

Defscape 16x 1.5
Defscape 32x 1.5
Defscape 64x 1.5

Thanks.