Your hinv shows "XGE24" graphics, which is what SGI renamed the XL24 card when used in conjunction with a R5000 CPU. (The enhanced floating point speed of the R5000 was intended to speed up 3D geometry calculations, so R5000+XL is quite different from R4000+XL in terms of 3D performance: to make that clearer they changed the name).
The XL24 card does not have any texture engine. It can draw flat shaded triangles quite fast from its modified-Bresenham circuitry that is used for both lines and triangles, but any textured poly must be drawn pixel-by-pixel by the CPU.
The other graphics option for the Indy, XZ (GR3-Elan) also does not have any texture engine, but it uses 4 Geometry Engines (GEs) to calculate 3D geometry instead of using the CPU. This is faster than a R4000 CPU but sometimes slower than a R5000. There was no SGI desktop graphics with texture hardware until the Indigo2 High/Max IMPACT in 1995.
You can see other comparisons of graphics performance on Ian Mapleson's site (sgidepot.co.uk). I'll link two that are especially relevant:
SGI Performance Comparisons (click "Inventor 3D (single buff)" to see results relevant to your query)
HolliDance Benchmark
Also note, if your "shaded triangle only" test is pegged at 60 FPS, you must be using double-buffering, which is not the best comparison for raw graphics performance (although it is usually required to prevent tearing). See
Why Double Buffering? from sgidepot again.
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