Another indication that the Catalyst 9.7 driver is buggy...
I ran PCIeSpeedTest_v0.1 with the Catalyst 9.7 driver. The cpu->gpu speed was 5.5 GB/s but the gpu->cpu speed was < 1 GB/s with the largest data sets. Same test with the Catalyst 9.5 driver shows 5.5 and 4.1 GB/s, respectively. (If it makes any difference, the mother board I have is MSI 790FX-GD70 and two HD 4870 are plugged in.)
For the time being, I am staying with the 9.5 driver.