The desc explicitly states this is the count of IPsec bytes decompression overhead — the processing overhead attributable to the decompression step. This quantifies the CPU cost side of IP compression rather than the bandwidth-savings side, so if this gateway is CPU-constrained, a large value here is one legitimate reason to consider disabling IPComp on that tunnel, even though doing so increases WAN bandwidth usage. e.g. cpvIpsecDecomprOverhead showing a meaningful chunk of decompression processing cost on a tunnel carrying a lot of already-compressible traffic, such as text-heavy application data.