It would not be terrible on XP. It all depends on what the machine is used for. For browsing only, you don't need more than XP. For office apps, windows 7 will allow some of the more recent app versions. CPU performance does matter. You can't just go by when the CPU came out. Benchmarks will give a clue. Regressing the OS is one of the ways to deal with performance issues. The newer the OS, more bloated it is in terms of background processes.Performance on a single decades old mechanical hard disk is going to be generally terrible
Here's how I look at performance. Slowest was XP CPU, next up I used for Windows 7 and Windows 10, Next up Windows 10. The fastest one is probably in the 50 percentile performance of all current PC's. It isn't fast but OK. At most i5-4570T, should run Windows 7 with SSD. Otherwise it's a struggle.
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