Your AGP graphics card slot speed will depend on the speed of the actual AGP slot in your system and the supporting AGP speed of the graphics card installed in it.

All AGP slots run on a 66MHz system bus which is twice the speed of the PCI slots. The original AGP 1x slot specification allowed a maximum data rate of 266MB/s, and as the AGP slot developed, faster 2x, then 4x and finally 8x AGP slots appeared. But the graphics card needed to still support the maximum speed of the slot, or it would clock down to the speed of the card, so a 4x AGP card in a 8x slot would still run at 4x speed.

All AGP slots still ran on a 66MHz bus speed, but for the faster AGP slot types the channel is double pumped for 2x, quad pumped for 4x and strobed eight times per clock for 8x AGP speeds, meaning that in theory the actual theoretical speed is actually 2x, 4x or 8x faster than the original 66MHz bus speed. This makes the 8x AGP speed a maximum of 2GB/s, which is roughly 4 times slower than PCI-E. Although also take into account that most motherboards, their north bridges, and the system ram at the time could not shift data this fast until quite late in the AGP slot's life.

For the age of your P3 I would guess it is using an AGP 2x slot and the graphics card is also a 2x speed specification. This means your graphics card is running on a double pumped 66MHz bus, effectively making it 133MHz, with a maximum datarate of 533MB/s.

Comparing this obsolete AGP 2x slot with the current PCI-E graphics slot, your P3's AGP slot is roughly 16 times slower at data transfer compared to the current standard.

BTW, did you know that early 1x and 2x AGP graphics cards are not compatible with later 4x and 8x slots? Putting such early cards in the later slots could completely fry the motherboards beyond repair due to a different core voltage being used in the two different specs.