newer windows releases are notably slower (yes…including 7…)
…
whereas ubuntu will feel way more responsive on low-spec hardware
This isn’t true. Windows 7 has better support for multi-threading applications, so these typically run faster on Win 7 than XP. Applications that do not support multi-threading generally run about the same speed, if there is a difference, it is marginal. In general terms, there is no significant difference on the overall performance (synthetic and real life benchmarks) between the two. The major issue with Win 7 over XP in that the battery life will be compromised slightly.
Regarding linux, although generally hard disk access is quicker, due to differences in the file system etc, synthetic benchmarks suggest that windows 7 for x86 outperforms unbuntu. (obvious caveat being that synthetic benchmarks don’t always translate into real world performance)