I'm about to buy a laptop specifically for software development and want to be sure I don't end up with something I'm unhappy with, as I have experienced the pain of waiting 2+ minutes when compiling code on an old i3 laptop.
I'm looking at something with an i7-7500u cpu and 256GB ssd which I thought would do the job admirably, but then I saw that the cpu only has 2 cores / 4 threads. (https://www.intel.co.uk/content/www/uk/en/products/processors/core/i7-processors/i7-7500u.html)
Is compiling (C# MVC web application) mainly based on single thread performance or is it likely that I'll see significant improvement spending a bit more and going for a 4 core / 8 thread cpu?
If the compiler would schedule all the CPU threads to build the program then it matters. If not, then I think there is not much benefit in increasing the CPU core/thread number. I think you may check the CPU usage when you compile the program and check if the usage of CPU is high enough. If it is below 50%, then I think it only uses one or two CPU thread to build the program, and increasing CPU core/thread count will not give you much benefit.
Also, if you use the hdd, not the ssd in your old laptop, then I think the bottleneck of the performance would probably be the read/write to disk drive, not the CPU. The usage of ssd would help a lot to improve the performance.