Heat causes aerial cables to soften, expand, sag, and get longer. Which increases latency slightly, and can increase packet loss (and thus retransmissions, decreasing effective throughput). This is true of both aerial fiber and aerial copper, but not of subterranean cable (which is thermally insulated, and supported along its length).
Heat also causes active devices to fail, if they’re not actively cooled, or that cooling fails or is insufficient, which causes traffic to be concentrated onto backup paths, leading to contention and congestion and TCP back-off.