Yep, my 4070 also slows down at the end:
Running Bench test on:NVIDIA GeForce RTX 4070
Chunk: 0 ( 0- 128)MB Speed: 435.37 GByte/s OK
Chunk: 1 ( 128- 256)MB Speed: 433.31 GByte/s OK
Chunk: 2 ( 256- 384)MB Speed: 439.11 GByte/s OK
Chunk: 3 ( 384- 512)MB Speed: 439.68 GByte/s OK
Chunk: 4 ( 512- 640)MB Speed: 438.94 GByte/s OK
Chunk: 5 ( 640- 768)MB Speed: 438.95 GByte/s OK
Chunk: 6 ( 768- 896)MB Speed: 435.53 GByte/s OK
Chunk: 7 ( 896-1024)MB Speed: 438.52 GByte/s OK
Chunk: 8 (1024-1152)MB Speed: 441.69 GByte/s OK
Chunk: 9 (1152-1280)MB Speed: 436.01 GByte/s OK
Chunk: 10 (1280-1408)MB Speed: 435.35 GByte/s OK
Chunk: 11 (1408-1536)MB Speed: 442.73 GByte/s OK
Chunk: 12 (1536-1664)MB Speed: 438.35 GByte/s OK
Chunk: 13 (1664-1792)MB Speed: 436.13 GByte/s OK
Chunk: 14 (1792-1920)MB Speed: 433.76 GByte/s OK
Chunk: 15 (1920-2048)MB Speed: 432.27 GByte/s OK
Chunk: 16 (2048-2176)MB Speed: 436.60 GByte/s OK
Chunk: 17 (2176-2304)MB Speed: 418.68 GByte/s OK
Chunk: 18 (2304-2432)MB Speed: 442.53 GByte/s OK
Chunk: 19 (2432-2560)MB Speed: 425.36 GByte/s OK
Chunk: 20 (2560-2688)MB Speed: 433.80 GByte/s OK
Chunk: 21 (2688-2816)MB Speed: 431.34 GByte/s OK
Chunk: 22 (2816-2944)MB Speed: 426.99 GByte/s OK
Chunk: 23 (2944-3072)MB Speed: 426.47 GByte/s OK
Chunk: 24 (3072-3200)MB Speed: 426.64 GByte/s OK
Chunk: 25 (3200-3328)MB Speed: 441.06 GByte/s OK
Chunk: 26 (3328-3456)MB Speed: 439.41 GByte/s OK
Chunk: 27 (3456-3584)MB Speed: 435.40 GByte/s OK
Chunk: 28 (3584-3712)MB Speed: 425.97 GByte/s OK
Chunk: 29 (3712-3840)MB Speed: 430.31 GByte/s OK
Chunk: 30 (3840-3968)MB Speed: 435.92 GByte/s OK
Chunk: 31 (3968-4096)MB Speed: 441.98 GByte/s OK
Chunk: 32 (4096-4224)MB Speed: 432.70 GByte/s OK
Chunk: 33 (4224-4352)MB Speed: 436.67 GByte/s OK
Chunk: 34 (4352-4480)MB Speed: 442.32 GByte/s OK
Chunk: 35 (4480-4608)MB Speed: 442.35 GByte/s OK
Chunk: 36 (4608-4736)MB Speed: 441.03 GByte/s OK
Chunk: 37 (4736-4864)MB Speed: 440.99 GByte/s OK
Chunk: 38 (4864-4992)MB Speed: 436.58 GByte/s OK
Chunk: 39 (4992-5120)MB Speed: 428.49 GByte/s OK
Chunk: 40 (5120-5248)MB Speed: 440.20 GByte/s OK
Chunk: 41 (5248-5376)MB Speed: 436.56 GByte/s OK
Chunk: 42 (5376-5504)MB Speed: 437.10 GByte/s OK
Chunk: 43 (5504-5632)MB Speed: 436.75 GByte/s OK
Chunk: 44 (5632-5760)MB Speed: 440.04 GByte/s OK
Chunk: 45 (5760-5888)MB Speed: 433.67 GByte/s OK
Chunk: 46 (5888-6016)MB Speed: 434.72 GByte/s OK
Chunk: 47 (6016-6144)MB Speed: 436.61 GByte/s OK
Chunk: 48 (6144-6272)MB Speed: 440.88 GByte/s OK
Chunk: 49 (6272-6400)MB Speed: 425.53 GByte/s OK
Chunk: 50 (6400-6528)MB Speed: 442.51 GByte/s OK
Chunk: 51 (6528-6656)MB Speed: 438.54 GByte/s OK
Chunk: 52 (6656-6784)MB Speed: 433.66 GByte/s OK
Chunk: 53 (6784-6912)MB Speed: 434.76 GByte/s OK
Chunk: 54 (6912-7040)MB Speed: 436.39 GByte/s OK
Chunk: 55 (7040-7168)MB Speed: 432.72 GByte/s OK
Chunk: 56 (7168-7296)MB Speed: 436.03 GByte/s OK
Chunk: 57 (7296-7424)MB Speed: 433.64 GByte/s OK
Chunk: 58 (7424-7552)MB Speed: 441.86 GByte/s OK
Chunk: 59 (7552-7680)MB Speed: 423.28 GByte/s OK
Chunk: 60 (7680-7808)MB Speed: 442.02 GByte/s OK
Chunk: 61 (7808-7936)MB Speed: 441.85 GByte/s OK
Chunk: 62 (7936-8064)MB Speed: 425.53 GByte/s OK
Chunk: 63 (8064-8192)MB Speed: 433.13 GByte/s OK
Chunk: 64 (8192-8320)MB Speed: 435.45 GByte/s OK
Chunk: 65 (8320-8448)MB Speed: 429.48 GByte/s OK
Chunk: 66 (8448-8576)MB Speed: 425.10 GByte/s OK
Chunk: 67 (8576-8704)MB Speed: 419.71 GByte/s OK
Chunk: 68 (8704-8832)MB Speed: 433.75 GByte/s OK
Chunk: 69 (8832-8960)MB Speed: 436.32 GByte/s OK
Chunk: 70 (8960-9088)MB Speed: 439.31 GByte/s OK
Chunk: 71 (9088-9216)MB Speed: 431.43 GByte/s OK
Chunk: 72 (9216-9344)MB Speed: 424.17 GByte/s OK
Chunk: 73 (9344-9472)MB Speed: 430.66 GByte/s OK
Chunk: 74 (9472-9600)MB Speed: 441.98 GByte/s OK
Chunk: 75 (9600-9728)MB Speed: 439.39 GByte/s OK
Chunk: 76 (9728-9856)MB Speed: 441.16 GByte/s OK
Chunk: 77 (9856-9984)MB Speed: 429.40 GByte/s OK
Chunk: 78 (9984-10112)MB Speed: 439.08 GByte/s OK
Chunk: 79 (10112-10240)MB Speed: 440.00 GByte/s OK
Chunk: 80 (10240-10368)MB Speed: 441.03 GByte/s OK
Chunk: 81 (10368-10496)MB Speed: 434.06 GByte/s OK
Chunk: 82 (10496-10624)MB Speed: 438.59 GByte/s OK
Chunk: 83 (10624-10752)MB Speed: 437.30 GByte/s OK
Chunk: 84 (10752-10880)MB Speed: 374.40 GByte/s OK
Chunk: 85 (10880-11008)MB Speed: 440.28 GByte/s OK
Chunk: 86 (11008-11136)MB Speed: 436.33 GByte/s OK
Chunk: 87 (11136-11264)MB Speed: 437.14 GByte/s OK
Chunk: 88 (11264-11392)MB Speed: 437.22 GByte/s OK
Chunk: 89 (11392-11520)MB Speed: 12.16 GByte/s OK
Chunk: 90 (11520-11648)MB Speed: 12.32 GByte/s OK
Chunk: 91 (11648-11776)MB Speed: 12.36 GByte/s OK
Chunk: 92 (11776-11904)MB Speed: 12.24 GByte/s OK
Chunk: 93 (11904-12032)MB Speed: 12.23 GByte/s OK
Chunk: 94 (12032-12160)MB Speed: 12.28 GByte/s OK