FWIW, a little experimented I just ran showed me that with simple layers the cold start time of my little 3MB Go app was <100ms, using the Docker image `amazon/aws-lambda-go:1` instead took ~1500ms.
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REPORT RequestId: f905d5fe-a64e-48c8-b1f2-6535640a6f82 Duration: 7.55 ms Billed Duration: 1309 ms Memory Size: 256 MB Max Memory Used: 49 MB Init Duration: 1301.10 ms
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REPORT RequestId: 89afb20d-bc49-4d89-91f0-f1ef62ac99aa Duration: 12.20 ms Billed Duration: 13 ms Memory Size: 256 MB Max Memory Used: 35 MB Init Duration: 85.37 ms
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REPORT RequestId: f905d5fe-a64e-48c8-b1f2-6535640a6f82 Duration: 7.55 ms Billed Duration: 1309 ms Memory Size: 256 MB Max Memory Used: 49 MB Init Duration: 1301.10 ms
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REPORT RequestId: 89afb20d-bc49-4d89-91f0-f1ef62ac99aa Duration: 12.20 ms Billed Duration: 13 ms Memory Size: 256 MB Max Memory Used: 35 MB Init Duration: 85.37 ms